BUG/MEDIUM: cfgcheck: verify existing log-forward listeners during config check

User reported that the config check returns an error with the message:
"Configuration file has no error but will not start (no listener) => exit(2)."
if the configuration present only a log-forward section with bind or dgram-bind
listeners but no listen/backend nor peer sections.

The process checked if there was 'peers' section avalaible with
an internal frontend (and so a listener) or a 'listen/backend'
section not disabled with at least one configured listener (into the
global proxies_list). Since the log-forward proxies appear in a
different list, they were not checked.

This patch adds a lookup on the 'log-forward' proxies list to check
if one of them presents a listener and is not disabled. And
this is done only if there was no available listener found into
'listen/backend' sections.

I have also studied how to re-work this check considering the 'listeners'
counter used after startup/init to keep the same algo and avoid further
mistakes but currently this counter seems increased during config parsing
and if a proxy is disabled, decreased during startup/init which is done
after the current config check. So the fix still not rely on this
counter.

This patch should fix the github issue #1346

This patch should be backported as far as 2.3 (so on branches
including the "log-forward" feature)

src/haproxy.c

@@ -2059,6 +2059,13 @@ static void init(int argc, char **argv)
 			if (!px->disabled && px->li_all)
 				break;
 
+		if (!px) {
+			/* We may only have log-forward section */
+			for (px = cfg_log_forward; px; px = px->next)
+				if (!px->disabled && px->li_all)
+					break;
+		}
+
 		if (pr || px) {
 			/* At least one peer or one listener has been found */
 			qfprintf(stdout, "Configuration file is valid\n");