PR 4146 (partial fix)
* tapsets.cxx (all_session_groups): Added note about stating that
begin probes should be registered (actually run) first and end
probes should be unregistered (run) last.
* translate.cxx (c_unparser::emit_module_exit): Process probe
group vector in reverse order so that probe groups will get
unregistered in the reverse order that they were registered.