Reject empty listening addresses

Using an empty listening address will result in chasquid listening on a
random port, which is a dangerous misconfiguration.

That is most likely done to prevent it from listening at all.

To prevent this misconfiguration, explicitly reject empty listening
addresses early and with a warning, so there is no ambiguity.

Users can still prevent chasquid from listening by just commenting out
the entry in the config (and not passing any systemd file descriptors).

See https://github.com/albertito/chasquid/issues/45 for more details and
discussion, including alternatives considered.

Thanks to xavierg who reported this via IRC.

test/t-20-bad_configs/run.sh

@@ -18,7 +18,7 @@ mkdir -p c-04-no_cert_dirs/certs/
 
 # Generate certs for the tests that need them.
 for i in c-05-no_addrs c-06-bad_maillog c-07-bad_domain_info \
-	c-08-bad_sts_cache c-09-bad_queue_dir ;
+	c-08-bad_sts_cache c-09-bad_queue_dir c-10-empty_listening_addr ;
 do
 	CONFDIR=$i/ generate_certs_for testserver
 done