MFC r269583:
Provide -o vers= support for mount_nfs.
Our mount_nfs does use -o nfsv<2|3|4> or -2 or -3 to specify the version.
OSX (these days), Solaris, and Linux use -o vers=<2,3,4>.
With the upcoming autofs support we can make a lot of (entrerprisy) setups
getting mount options from LDAP just work by providing -o vers= compatibility.
Reviewed by: wblock, bjk (man page), rmacklem, emaste
Sponsored by: DARPA,AFRL
PR: 192379
git-svn-id: svn://svn.freebsd.org/base/stable/9@271508
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