text-based mount.nfs: add function to do foreground retries

Make the differences between the foreground and background mount logic
explicit by creating separate functions to handle each -- think of them as
separate scripts for doing a foreground or a background mount.

NFS foreground mounts are supposed to retry for a little while before
giving up.  Add a function to handle this.

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