[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.

src/haproxy.c

@@ -185,6 +185,9 @@ void display_build_opts()
 	       "\n  maxconn = %d, bufsize = %d, maxrewrite = %d, maxpollevents = %d"
 	       "\n\n",
 	       DEFAULT_MAXCONN, BUFSIZE, MAXREWRITE, MAX_POLL_EVENTS);
+
+	list_pollers(stdout);
+	putchar('\n');
 }
 
 /*