text-based mount.nfs: Add functions to handle background mounting

Add helper functions that handle background mounts; one each for
foreground processing (to try the request, and determine when to fork);
and one for background processing (retry the request multiple times as
a forked background daemon).

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