Every now and then we see issues arising from a missing gpg-agent. It may fail to start again after "restart background processes" (or "gpgconf --kill all") e.g. if there are no certificates in the certificate list. But there are other occurrences as well.

I think we should not rely on gpg starting the agent again anyway but do it explicitly when doing "restart background processes".

This is a follow up to T5775: Kleopatra: Add action to restart background processes. It probably would make T6567: Kleopatra: gpg-agent does not restart when trying to access smart card obsolete, as Kleopatra users usually do not stop processes with the Cli we only need to cover "restart background processes" here.