BUG/MINOR: haproxy: Fix option to disable the fast-forward

The option was renamed to only permit to disable the fast-forward. First
there is no reason to enable it because it is the default behavior. Then it
introduced a bug because there is no way to be sure the command line has
precedence over the configuration this way. So, the option is now named
"tune.disable-fast-forward" and does not support any argument. And of
course, the commande line option "-dF" has now precedence over the
configuration.

No backport needed.
diff --git a/src/haproxy.c b/src/haproxy.c
index 804e092..3025d5a 100644
--- a/src/haproxy.c
+++ b/src/haproxy.c
@@ -1598,6 +1598,8 @@
 #endif
 	global.tune.options |= GTUNE_STRICT_LIMITS;
 
+	global.tune.options |= GTUNE_USE_FAST_FWD; /* Use fast-forward by default */
+
 	/* keep a copy of original arguments for the master process */
 	old_argv = copy_argv(argc, argv);
 	if (!old_argv) {
@@ -1649,7 +1651,7 @@
 				global.tune.options &= ~GTUNE_USE_REUSEPORT;
 #endif
 			else if (*flag == 'd' && flag[1] == 'F')
-				global.tune.options |= GTUNE_NO_FAST_FWD;
+				global.tune.options &= ~GTUNE_USE_FAST_FWD;
 			else if (*flag == 'd' && flag[1] == 'V')
 				global.ssl_server_verify = SSL_SERVER_VERIFY_NONE;
 			else if (*flag == 'V')