vfs: Permit unix sockets to be opened with O_PATH

As with FIFOs, a path descriptor for a unix socket cannot be used with
kevent().

In principle connectat(2) and bindat(2) could be modified to support an
AT_EMPTY_PATH-like mode which operates on the socket referenced by an
O_PATH fd referencing a unix socket. That would eliminate the path
length limit imposed by sockaddr_un.

Update O_PATH tests.

Reviewed by: kib
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D31970