Enable GnuPG system sockets

This makes use of gnupg!2 (merged) and results in GnuPG's daemons being supervised instead of getting spawned into random cgroups.

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.