Modify the nfsuserd daemon so that it uses an AF_LOCAL socket for upcalls.

This patch modifies the nfsuserd daemon so that it uses an AF_LOCAL socket
for upcalls by default. This should fix the problem with using a UDP
socket upcall to 127.0.0.1 when jails are used.
The AF_LOCAL socket case only supports a single server daemon, since hangs
were observed by the original problem reporter when multiple daemons
were used.
The patch adds a command line option called "-use-udpsock" which makes
the daemon revert to its prepatched behaviour.

Suggested by: dfr
PR: 205193
Relnotes: yes