Rename vfs.nfsd.enable_uidtostring to vfs.nfs.enable_uidtostring.
It applies to both NFS client and NFS server, and is useful for both.
This is different from vfs.nfsd.enable_stringtouid, which is specific
to server side.

Reviewed by: rmacklem@
MFC after: 2 weeks
Sponsored by: DARPA, AFRL