Switch to systemd activation in D-Bus service definitions

This addresses the last piece of #41 by preventing D-Bus from running its own command to start gnome-keyring-daemon and instead having it delegate to the existing systemd user unit when supported. See individual commit messages for more details. The question of whether all these dbus services should be installed unconditionally (instead of managed by an alternatives system) is still open as #86, but this change is orthogonal: it doesn't make that situation any better or worse than it is today.