nsenter: Allow selecting the uid and gid to be used in the entered userns

Using -S (--setuid) and -G (--setgid) one can select the uid/gid which
will be used in the entered user namespace.

[kzak@redhat.com: - use setuid/gid unconditionally (always),
                  - update man page]

Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Karel Zak <kzak@redhat.com>