Make it possible to mount nullfs(5) using plain mount(8)
instead of mount_nullfs(8).

Obviously you'd need to force mount(8) to not call
mount_nullfs(8) to make use of it.

Reviewed by: kib
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D26934