Redo systemd scope tracking

We won't reliably get a pid anymore, so redo unit path finding by using
the scope's name directly and completely strip out the fallback path.
Completely drop SSH_AGENT_PID

Upstream gpg-agent doesn't provide it anymore, we don't really need it
anyways.
Only print GPG_AGENT_INFO if provided
Default to standard sockets

Upstream gpg-agent is now defaulting to these standard sockets and then
suppressing the environmental variables. If we don't get it from
envoyd, assume its a standard socket.
Add a kill command to envoyd

Since envoyd no longer concerns itself with the agent's pid, message
envoyd directly over IPC to kill the agent.
Better error messages
Merge branch 'gnupg-2.1.0'