nsenter: allow to use --set{uid,gid} for all namespaces

Now it's possible to set UID and GID for user namespaces only. This
patch removes this restriction and allow to use --set{uid,gid} in all
cases. The default for user namespaces is still GID=0, UID=0.

Reported-by: Tomas Doran <bobtfish@bobtfish.net>
Signed-off-by: Karel Zak <kzak@redhat.com>