BUG/MINOR: log-forward: fail on unknown keywords

The log-forward section silently ignores junk and unknown keywords, make
it fail! No backport is needed.
diff --git a/src/log.c b/src/log.c
index 5f1f96e..e8b172d 100644
--- a/src/log.c
+++ b/src/log.c
@@ -3688,6 +3688,11 @@
 			goto out;
 		}
 	}
+	else {
+		ha_alert("parsing [%s:%d] : unknown keyword '%s' in log-forward section.\n", file, linenum, args[0]);
+		err_code |= ERR_ALERT | ERR_ABORT;
+		goto out;
+	}
 out:
 	return err_code;
 }