nfsidmap: Allow a particular key to be revoked.

Introducing three new command line arguments
that allow particular keys to be revoke

  -u will remove a uid key
  -g will revoke a gid key
  -r will revoke both the uid and gid keys

A user name has also needs to be supply with
these new flags.

Signed-off-by: Steve Dickson <steved@redhat.com>