MFC: r212125, r212126
Modify lib/libstand/nfs.c to use NFSv3 instead of NFSv2.
This allows the nfs_getrootfh() function to return the
correct file handle size to pxe.c for pxeboot. It also
results in NFSv2 no longer being used by default anywhere
in FreeBSD. If built with OLD_NFSV2 defined, the old
code that predated this patch will be built and NFSv2
will be used. pxe.c is also modified to use this version
of nfs_getrootfh() so that pxeboot will use NFSv3 and
work for non-FreeBSD as well as FreeBSD NFS servers.