Add system service templates

These can provide supervised services for /etc/pacman.d/gnupg, preventing the daemons from being spawned into random cgroups. This is utilized by pacman!4 (merged).

For example, when archlinux-keyring is updated, gpg-agent is activated. The agent does not shut down by itself, keeping the user session in closing state forever. This unclosed session then prevents another local user from shutting down the system without admin rights.