PR11246 cont'd: Add options for cache control

PR11246 cont'd: Add options for cache control

--disable-cache : turn off all caching
--clean-cache   : clean up stale entries and then quit
--poison-cache  : force regeneration of items that would have hit the cache

These are undocumented for now, until we decide whether they are
generally useful.

* main.cxx (main): Parse the new options.
* session.h (systemtap_session): Add poison_cache; document the others.
* clean.cxx (clean_cache): No longer static.
  (get_stapconf_from_cache, get_script_from_cache): Respect poison.
* tapsets.cxx (tracepoint_builder::get_tracequery_module): Ditto.
  (dwarf_cast_expanding_visitor::filter_special_modules): Ditto.