Set*id system calls

SetXid programs are running with effective uid/gid, but original uid/gid are still known to the program.  Thus a setXid program can easily get back to the original uid/gid settings.

Useful system calls include:
setuid( uid );
setgid( gid );
Set real id (and effective/saved/fs iff root)

setreuid( ruid,  euid );
setregid( rgid,  egid );
set real and effective ids (can use -1 to indicate no change)

setresuid( ruid, euid, suid );
setresgid( rgid, egid, sgid );
set real, effective, and saved ids (can use -1 to indicate no change)