Fix mount_nfs so that it doesn't create mounttab entries for NFSv4 mounts.

The NFSv4 protocol doesn't use the Mount protocol, so it doesn't make sense
to add an entry for an NFSv4 mount to /var/db/mounttab. Also, r308871
modified umount so that it doesn't remove any entry created by mount_nfs.
Reported on freebsd-current@.

Reported by: clbuisson@orange.fr
MFC after: 2 weeks