Change the default uid/gid values for nobody/nogroup to 65534/65533.

The default values found in /etc/passwd and /etc/group are 65534, 65533.
In mountd.c, the defaults were -2, which was 65534 back when uid_t was 16bits.
Without this patch, a file created by root on an NFS exported volume without
the "-root=" export option will end up owned by uid 4**32 - 2.
When discussed on freebsd-current@, it seemed that users preferred the
values being changed to 65534/65533.
I have not added code to acquire these values from the databases, since
the mountd daemon might get "stuck" during startup waiting for a non-responsive
password database server.

Discussed on: freebsd-current