mount.nfs: Add new files for supporting string-ified mount options

Introduce support files which contain code that builds string mount
options and passes them to the kernel.  This is a pre-requisite for
actually enabling /sbin/mount.nfs to do text-based mounts.

This is only partially complete at the moment, but is presented so that
folks can start banging on the kernel mount option string parsing code.
There are clearly still parts that are not implemented quite yet, such
as bg and retry support, but it should be enough to get going.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Neil Brown <neilb@suse.de>