Reorganize cache and temporary directories under common parents.

Previously, various cache and temporary directories were sprinkled all
over ~/.mythtv, aka MYTHCONFDIR.  This change puts them all under
~/.mythtv/cache or ~/.mythtv/tmp, though, there could still be a few
I've missed.  Some of these directories can get quite large and having
them all in more centralized locations makes it easier to track how
much space they use and to not back them up.