Revert "MINOR: mux-quic: Disable zero-copy forwarding for send by def…

…ault"

This reverts commit 18f2ccd.

Found issues related to QUIC fast-forward were resolved (see github
issue haproxy#2372). Reenable it by default. If any issue arises, it can be
disabled using the global statement :
  tune.quit.zero-copy-fwd-send off

This can be backported to 2.9, but only after a sensible period of
observation.

doc/configuration.txt

@@ -3652,7 +3652,7 @@ tune.quic.socket-owner { connection | listener }
 
 tune.quic.zero-copy-fwd-send { on | off }
   Enables ('on') of disabled ('off') the zero-copy sends of data for the QUIC
-  multiplexer. It is disabled by default.
+  multiplexer. It is enabled by default.
 
   See also: tune.disable-zero-copy-forwarding
 

src/haproxy.c

@@ -1601,7 +1601,7 @@ static void init_args(int argc, char **argv)
 	global.tune.options |= GTUNE_USE_FAST_FWD; /* Use fast-forward by default */
 
 	/* Use zero-copy forwarding by default */
-	global.tune.no_zero_copy_fwd = NO_ZERO_COPY_FWD_QUIC_SND;
+	global.tune.no_zero_copy_fwd = 0;
 
 	/* keep a copy of original arguments for the master process */
 	old_argv = copy_argv(argc, argv);