MINOR: config: Add a warning if tune.chksize is used

This option is now deprecated. It is recent, but it is now marked as
deprecated as far as 2.2. Thus, there is now a warning in the 2.4 if this
option is still used. It will be removed in 2.5.

Becaue the 2.3 is quite new, this patch may be backported to 2.3.
diff --git a/src/cfgparse-global.c b/src/cfgparse-global.c
index ca447bb..ba7249e 100644
--- a/src/cfgparse-global.c
+++ b/src/cfgparse-global.c
@@ -190,7 +190,8 @@
 		global.tune.maxaccept = max;
 	}
 	else if (!strcmp(args[0], "tune.chksize")) {
-		/* Deprecated now */
+		ha_warning("parsing [%s:%d]: the option '%s' is deprecated and will be removed in next version.\n",
+			   file, linenum, args[0]);
 	}
 	else if (!strcmp(args[0], "tune.recv_enough")) {
 		if (alertif_too_many_args(1, file, linenum, args, &err_code))