stage1: generate socket unit for socket-activated ports

 * Generate a systemd socket unit for socket-activated ports
 * Add socket-related path generators and cleanup rest
 * Remove unnecessary systemd unit path mkdirs, just trust the
   stage1 rootfs provides them.
imageID is so general, these are really the app-imageID's, it seems more contextually relevant to say appID than imageID, these are after all the imageIDs for the apps listed in ContainerRuntimeManifest.Apps.  appImageID seemed too lengthy, but maybe that would be better.  Maybe from a consistency standpoint imageID makes more sense, *shrug*.  No strong feelings here.