umount.nfs command: Add an AF_INET6-capable version of nfs_call_unmount()

We need an AF_INET6-capable version of nfs_call_unmount() to allow the
umount.nfs command to support unmounting NFS servers over IPv6.  The
legacy
mount.nfs command still likes to use nfs_call_umount(), so we leave it
in
place and introduce a new API that can take a "struct sockaddr *".

The umount.nfs command will invoke this new API, but we'll leave the
legacy
mount.nfs command and the umount.nfs4 command alone.  The umount.nfs4
command does not need this support because NFSv4 unmount operations are
entirely local.

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