varlink: also honour new env var $SYSTEMD_VARLINK_LISTEN in varlink_server_listen_auto()

varlink: also honour new env var $SYSTEMD_VARLINK_LISTEN in varlink_server_listen_auto()

varlink_server_listen_auto() is supposed to be the one-stop solution for
turning simple command line tools into IPC services. They aren't easy to
test/debug however, since you have to invoke them through a service
manager.

Let's make this easier: if the SYSTEMD_VARLINK_LISTEN env var is set,
let's listen on the socket specified therein. This makes things easier
to gdb: just run the service from the cmdline.