Add man pages for seteuid() and friends.

Also add an overview page that explains the general concepts.

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

@@ -0,0 +1,38 @@
+## 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)