Shorten user service updates

The user unit links through result-service, so each build replaces its
package without another unit link. The update command builds with that
out-link, reloads the user manager, and uses try-restart to restart only
an active service. A stopped service stays stopped.

The README gives the first link and the two update commands. The checks
include shell lint for the update script.

Issue: 99a5e07