Set default uid/gid to nobody/nogroup for NFSv4 mapping.

The default uid/gid for NFSv4 are set by the nfsuserd(8) daemon.
However, they were 0 until the nfsuserd(8) was run. Since it is
possible to use NFSv4 without running the nfsuserd(8) daemon, set them
to nobody/nogroup initially.
Without this patch, the values would be set by the nfsuserd(8) daemon
and left changed even if the nfsuserd(8) daemon was killed. Also, the default
values of 0 meant that setting a group to "wheel" would fail even when
done by root and this patch fixes this issue.

MFC after: 2 weeks