add missing long option for -r in mountd

I forgot a bit of code that needs to go into patch 3 that I posted yesterday.
This adds a long option for the -r option. I'll post manpage update patches
once I get them written up.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Neil Brown <neilb@suse.de>