umount.nfs command: Support AF_INET6 server addresses

Replace existing mount option parser in nfsumount.c with the new pmap
stuffer
function nfs_options2pmap().  Mount option parsing for umount.nfs now
works
the same as it does for mount option rewriting in the text-based
mount.nfs
command.

This adds a number of new features:

  1.  The new logic supports resolving AF_INET6 server addresses
  2.  Support is added for the recently introduced "mountaddr" option.
  3.  Parsing numeric option values is much more careful
  4.  Option parsing no longer uses xmalloc/xstrdup, so it won't fail
      silently if memory can't be allocated
  5.  Mount program number set in /etc/rpc is respected
  6.  Mount doesn't exit with EX_USAGE if the hostname lookup fails

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Steve Dickson <steved@redhat.com>