MFC: r265252

MFC: r265252
The new draft specification for NFSv4.0 specifies that a server
should either accept owner and owner_group strings that are just
the digits of the uid/gid or return NFS4ERR_BADOWNER.
This patch adds a sysctl vfs.nfsd.enable_stringtouid, which can
be set to enable the server w.r.t. accepting numeric string. It
also ensures that NFS4ERR_BADOWNER is returned if numeric uid/gid
strings are not enabled. This fixes the server for recent Linux
nfs4 clients that use numeric uid/gid strings by default.

git-svn-id: svn://svn.freebsd.org/base/stable/9@265724 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f