Add man pages for seteuid() and friends.

Also add an overview page that explains the general concepts.

Base/usr/share/man/man2/getegid.md

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+geteuid.md

Base/usr/share/man/man2/geteuid.md

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+## Name
+
+geteuid, getegid - get effective user / group id
+
+## Synopsis
+
+```**c++
+#include <unistd.h>
+
+uid_t geteuid(void);
+gid_t getegid(void);
+```
+
+## Description
+
+Returns the effective user or group id.
+
+## See also
+
+* [`setuid_overview`(7)](../man7/setuid_overview.md)
+* [`getuid`(2) / `getgid`(2)](getuid.md)
+* [`getresuid`(2) / `getresgid`(2)](getresuid.md)
+* [`seteuid`(2) / `setegid`(2)](seteuid.md)
+* [`setuid`(2) / `setgid`(2)](setuid.md)
+* [`setresuid`(2) / `setresgid`(2)](setresuid.md)

Base/usr/share/man/man2/getgid.md

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+getuid.md

Base/usr/share/man/man2/getresgid.md

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+getresuid.md

Base/usr/share/man/man2/getresuid.md

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+## Name
+
+getresuid, getresgid - get real, effective, and saved user / group ID
+
+## Synopsis
+
+```**c++
+#include <unistd.h>
+
+int getresuid(uid_t*, uid_t*, uid_t*);
+int getresgid(gid_t*, gid_t*, gid_t*);
+```
+
+## Description
+
+Returns the real, effective, and saved user or group ID.
+
+## Return value
+
+If the call was set successful, returns 0.
+Else, returns -1 and sets `errno` to describe the error.
+
+## Errors
+
+* `EFAULT`: One of the passed pointers is not valid, writeable pointer in the current process.
+
+## See also
+
+* [`setuid_overview`(7)](../man7/setuid_overview.md)
+* [`geteuid`(2) / `getegid`(2)](geteuid.md)
+* [`getuid`(2) / `getgid`(2)](getuid.md)
+* [`seteuid`(2) / `setegid`(2)](seteuid.md)
+* [`setuid`(2) / `setgid`(2)](setuid.md)
+* [`setresuid`(2) / `setresgid`(2)](setresuid.md)

Base/usr/share/man/man2/getuid.md

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+## Name
+
+getuid, getgid - get real user / group id
+
+## Synopsis
+
+```**c++
+#include <unistd.h>
+
+uid_t getuid(void);
+gid_t getgid(void);
+```
+
+## Description
+
+Returns the real user or group id.
+
+## See also
+
+* [`setuid_overview`(7)](../man7/setuid_overview.md)
+* [`geteuid`(2) / `getegid`(2)](geteuid.md)
+* [`getresuid`(2) / `getresgid`(2)](getresuid.md)
+* [`seteuid`(2) / `setegid`(2)](seteuid.md)
+* [`setuid`(2) / `setgid`(2)](setuid.md)
+* [`setresuid`(2) / `setresgid`(2)](setresuid.md)

Base/usr/share/man/man2/setegid.md

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+seteuid.md

Base/usr/share/man/man2/seteuid.md

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+## Name
+
+seteuid, setegid - set effective user / group ID
+
+## Synopsis
+
+```**c++
+#include <unistd.h>
+
+int seteuid(uid_t);
+int setegid(gid_t);
+```
+
+## Description
+
+Sets the effective user or group ID.
+
+For non-superusers, the effective ID can only be set to the current real or saved ID.
+
+In particular, `seteuid(geteuid())` will fail if the current effective user ID is not equal to the current real or saved ID.
+
+## Return value
+
+If the call was set successful, returns 0.
+Else, returns -1 and sets `errno` to describe the error.
+
+## Errors
+
+* `EPERM`: The new ID is not equal to the real ID or saved ID, and the user is not superuser.
+
+## See also
+
+* [`setuid_overview`(7)](../man7/setuid_overview.md)
+* [`geteuid`(2) / `getegid`(2)](geteuid.md)
+* [`getuid`(2) / `getgid`(2)](getuid.md)
+* [`getresuid`(2) / `getresgid`(2)](getresuid.md)
+* [`setuid`(2) / `setgid`(2)](setuid.md)
+* [`setresuid`(2) / `setresgid`(2)](setresuid.md)

Base/usr/share/man/man2/setgid.m

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+setuid.md

Base/usr/share/man/man2/setresgid.md

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+setresuid.md

Base/usr/share/man/man2/setresuid.md

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+## Name
+
+setresuid, setresgid - set real, effective, and saved user / group ID
+
+## Synopsis
+
+```**c++
+#include <unistd.h>
+
+int setresuid(uid_t, uid_t, uid_t);
+int setresgid(gid_t, gid_t, gid_t);
+```
+
+## Description
+
+Sets all of real, effective, and saved user or group ID to the given values.
+
+An argument of `-1` keeps the corresponding ID unchanged.
+
+For non-superusers, each of the given values has to be equal to any of the current real, effective, or saved IDs for the call to succeed.
+
+## Return value
+
+If the call was set successful, returns 0.
+Else, returns -1 and sets `errno` to describe the error.
+
+## Errors
+
+* `EPERM`: At least one of the passed IDs was not equal to the current real, effective, or saved ID, and the user is not superuser.
+
+## See also
+
+* [`setuid_overview`(7)](../man7/setuid_overview.md)
+* [`geteuid`(2) / `getegid`(2)](geteuid.md)
+* [`getuid`(2) / `getgid`(2)](getuid.md)
+* [`getresuid`(2) / `getresgid`(2)](getresuid.md)
+* [`seteuid`(2) / `setegid`(2)](seteuid.md)
+* [`setuid`(2) / `setgid`(2)](setuid.md)

Base/usr/share/man/man2/setuid.md

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+## Name
+
+setuid, setgid - set user / group ID
+
+## Synopsis
+
+```**c++
+#include <unistd.h>
+
+int setuid(uid_t);
+int setgid(gid_t);
+```
+
+## Description
+
+Sets all of real, effective, and saved user or group ID to the given ID.
+
+For non-superusers, the given ID has to be equal to the current real or effective ID for the call to succeed.
+
+## Return value
+
+If the call was set successful, returns 0.
+Else, returns -1 and sets `errno` to describe the error.
+
+## Errors
+
+* `EPERM`: The new ID is not equal to the real ID or effective ID, and the user is not superuser.
+
+## See also
+
+* [`setuid_overview`(7)](../man7/setuid_overview.md)
+* [`geteuid`(2) / `getegid`(2)](geteuid.md)
+* [`getuid`(2) / `getgid`(2)](getuid.md)
+* [`getresuid`(2) / `getresgid`(2)](getresuid.md)
+* [`seteuid`(2) / `setegid`(2)](seteuid.md)
+* [`setresuid`(2) / `setresgid`(2)](setresuid.md)