Handle multiple python probes at the same location.

Handle multiple python probes at the same location.

* python/HelperSDT/__init__.py (Dispatcher.pytrace_dispatch): Handle
  multiple breakpoints at the same breakpoint location.
  (Dispatcher.do_b): Make sure the line number field is numeric. Clear the
  line number field if we've got a return break.
* tapset-python.cxx (python_builder::build): Add newlines to debug prints.