getsubids: system binary for user's sub*ids

getsubids: system binary for user's sub*ids

Rename list_subid_ranges to getsubids to provide a system binary to
check the sub*ids of a user. The intention is to provide this binary
with any distribution that includes the subid feature, so that system
administrators can check the subid ranges of a given user.

Finally, add a man page to explain the behaviour of getsubids.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1980780

Signed-off-by: Iker Pedrosa <ipedrosa@redhat.com>