Add support for AF_LOCAL socket upcalls to the nfsuserd daemon.

This patch adds support for AF_LOCAL socket upcalls to an nfsuserd daemon
that supports them. A future patch to the nfsuserd daemon will use AF_LOCAL
sockets to avoid a problem when using upcalls to 127.0.0.1 if jails are
in use.

Suggested by: dfr
PR: 205193