nullfs: add nounixbypass mount option

The option, when set, disables bypassing the unix socket vnode down
to the lower mp, effectively preventing connection to nullfs unix
socket from being acceptable from the lower mp (and vice versa).

This is done by providing a vop vector that stops bypass for unp-related
VOPs. I believe that VFS_VOP_VECTOR_REGISTER() does the right thing
there regardless of the order of initialization.

Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D52983