MFC r269583:

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/10@270043 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f