Reorganize iterate_over_labels

Reorganize iterate_over_labels

I noticed that iterate_over_labels was using a static variable as a
recursion variable, which isn't a safe thing to do since it will only be
initialized once.  While fixing that, I also reorganized the function
quite a bit.

* dwflpp.cxx (dwflpp::iterate_over_labels): Take the current function as
  a parameter instead of using a static local.  Rewrite some of the code
  as well to try to make it more obvious.
* tapsets.cxx (add_label_name): Remove in favor of query_label.
  (query_label): New, to check decl_file and fix probe listing.
  (query_srcfile_label, query_cu): Adjust to iterate_over_labels change
  and start using query_label as the callback.