daemon: Fix admin backend spawning

pkexec fails on some systems with "Refusing to render service to dead
parents.", which is caused by double forking when spawning the process.
Let's prevent this by G_SPAWN_DO_NOT_REAP_CHILD flag and clean up
manually using g_child_watch_add.

https://bugzilla.gnome.org/show_bug.cgi?id=793445