b2/: make the match name after "break" and "continue" optional

If the name is omitted, we jump back / out of the current block.

b2/SUBST

@@ -8,7 +8,7 @@ BAR=x	/* BAR= wouldn't be syntactically correct. We need a non-empty value */
 FOO=(*)(?) {
 	BAR=$BAR$2
 	FOO=$1
-	continue FOO
+	continue
 }
 BAR=x(*) { $=$1 }	/* remove the "x" */