service: Allow services to be run as root

There are some situations where a service has to be run as root, so add gss_service_run_with_flags() and define the flag GSS_SERVICE_RUN_FLAG_ALLOW_ROOT so the user of the API can communicate if the service should be allowed to run as root.

The specific situation this is meant for is a service running as root to avoid being killed when systemd gains control after the pivot to the final root filesystem (all non-root processes are killed at that point). But perhaps there are other situations where root is needed which aren't as hack-y.