MFC: r212123
Modify nfs_diskless.c to recognize the environment variable
boot.nfsroot.nfshandlelen and set the diskless root fs to
use NFSv3 and this file handle length when it is set. If
this environment variable is not set, the diskless root fs
will use NFSv2 and the same defaults as before. This fixes
the problem where the diskless nfs root fs had to be on a
FreeBSD server for NFSv3 to work, because it did not know
the correct file handle length and assumed the size used
by FreeBSD.