open-isns: systemd integration

Implement code to fetch sockets from systemd, and add socket
and service files.

Signed-off-by: Hannes Reinecke <hare@suse.de>
open-isns: remove unused SLPLIN from Makefile.in

Signed-off-by: Hannes Reinecke <hare@suse.de>
open-isns: Add install Makefile target

Signed-off-by: Hannes Reinecke <hare@suse.de>
Update spec file to install isns systemd files

Signed-off-by: Hannes Reinecke <hare@suse.de>
Update changelog