setgid sets the effective group ID of the current process. If the caller
is the superuser, the real and saved group ID's are also set.
Under Linux, setgid is implemented like the POSIX version with the
_POSIX_SAVED_IDS feature. This allows a setgid program that is not suid root
to drop all of its group privileges, do some un-privileged work, and then
re-engage the original effective group ID in a secure manner.