MINOR: cfgparse: warn when gid parameter is not a number

Currently, no warning are emitted when the gid is not a number.
Purpose of this warning is to let admins know they their configuration
won't be applied as expected.
diff --git a/src/cfgparse.c b/src/cfgparse.c
index 1a2c1ea..92f3611 100644
--- a/src/cfgparse.c
+++ b/src/cfgparse.c
@@ -1095,7 +1095,11 @@
 			err_code |= ERR_ALERT | ERR_FATAL;
 			goto out;
 		}
-		global.gid = atol(args[1]);
+		if (strl2irc(args[1], strlen(args[1]), &global.gid) != 0) {
+			Warning("parsing [%s:%d] :  gid: string '%s' is not a number.\n   | You might want to use the 'group' parameter to use a system group name.\n", file, linenum, args[1]);
+			err_code |= ERR_WARN;
+			goto out;
+		}
 	}
 	else if (!strcmp(args[0], "external-check")) {
 		if (alertif_too_many_args(0, file, linenum, args, &err_code))