[Feature Request]: Can the config entry for socket_path expand env variables

[lespea]

Right now, at least as far as I can tell, there's no way to have [daemon]'s socket_path resolve to different paths on different machines. This makes using systemd_socket difficult to use if the config is shared between machines since you can't reference $XDG_RUNTIME_DIR. Also maybe it would make sense to change the default socket_path if systemd_socket is set since most likely the user will want the socket in $XDG_RUNTIME_DIR?