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