mount.nfs: Enable mount.nfs to do text-based mount support

A new command line option, "-i", is added to mount.nfs to force the use of
the string interface for testing purposes.  "-s", "-t", and "-r" are
already taken or have legacy meaning so I picked "-i".

At some later point, when everyone is comfortable with the string mount
option parsing implementation, we will add a switch based on kernel
version, and remove the "-i" command line option.  For now, I am more
comfortable enabling it by hand instead.

Since this is a temporary arrangement, I'm leaving the option undocumented
in the mount.nfs man page.

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