[MINOR] report list of supported pollers with -vv

During troubleshooting, it's often useful to get the list of supported
pollers but until now it was required to have a working configuration
first. Since the pollers are known before main() is called, let's list
them with the build options.

Also report the default MAXCONN setting.

(cherry picked from commit be5b68584e)

src/haproxy.c

@@ -183,7 +183,13 @@ void display_build_opts()
 #ifdef BUILD_OPTIONS
 	       "\n  OPTIONS = " BUILD_OPTIONS
 #endif
-	       "\n\n");
+	       "\n\nDefault settings :"
+	       "\n  maxconn = %d, maxpollevents = %d"
+	       "\n\n",
+	       DEFAULT_MAXCONN, MAX_POLL_EVENTS);
+
+	list_pollers(stdout);
+	putchar('\n');
 }
 
 /*