Add vfs.nfsd.nfsd_enable_uidtostring, which works just like
vfs.nfsd.nfsd_enable_stringtouid, but in reverse - when set to 1,
it forces the NFSv4 server to return numeric UIDs and GIDs instead
of "user@domain" strings. This helps with clients that can't
translate returned identifiers, eg when rerooting.

The same can be achieved by just never running nfsuserd(8),
but the sysctl is useful to toggle the behaviour back and forth
without rebooting.

Reviewed by: rmacklem (earlier version)
MFC after: 2 weeks
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D11326