unionfs: implement VOP_UNP_* and remove special VSOCK vnode handling

unionfs has a bunch of clunky special-case code to avoid creating
unionfs wrapper vnodes for AF_UNIX sockets. This was added in 2008
to address PR 118346, but in the intervening years the VOP_UNP_*
operations have been added to provide a clean interface to allow
sockets to work in the presence of stacked filesystems.

PR: 275871
Reviewed by: kib (prior version), olce
Tested by: Karlo Miličević <karlo98.m@gmail.com>
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D44288