BUG/MINOR: tools: fix parsing "us" unit for timers

Commit c20ad0d8dbd1bb5707bbfe23632415c3062e046c (BUG/MINOR: tools:  make
parse_time_err() more strict on the timer validity) broke parsing the "us"
unit in timers. It caused `parse_time_err()` to return the string "s",
which indicates an error.

Now if the "u" is followed by an "s" we properly continue processing the
time instead of immediately failing.

This fixes #1209. It must be backported to all stable versions.
diff --git a/src/tools.c b/src/tools.c
index 546ad44..4924ad1 100644
--- a/src/tools.c
+++ b/src/tools.c
@@ -2326,6 +2326,7 @@
 		if (text[1] == 's') {
 			idiv = 1000000;
 			text++;
+			break;
 		}
 		return text;
 	case 'm': /* millisecond : "ms" or minute: "m" */