Add man pages for seteuid() and friends.

Also add an overview page that explains the general concepts.

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

@@ -0,0 +1,36 @@
+## 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)