Port ssh-agent from gnome-keyring

This series turns the ssh-agent wrapper, currently hosted as a sub-daemon of gnome-keyring-daemon, into a standalone binary in gcr (/usr/libexec/gcr-ssh-agent). That would make the service manageable by systemd.

The gcr-ssh-agent executable takes a base directory of sockets: the one pointed by SSH_AUTH_SOCK and the other that is used by internally spawned ssh-agent. It shall be socket activated through the former socket, but I'm not completely sure how the systemd unit file should look like in this case.

See the background information in the planned Fedora Change: https://fedoraproject.org/wiki/Changes/ModularGnomeKeyring