core: cleanup config dir/file handling

[Morg42]

creating/guessing config dirs and config files has been quite wild (and not always consistent) throughout shng, and not all methods were prepared to honor extern_conf_dir or config_etc.

• lib.smarthome has new methods:
  • get_config_dir(config)
  • get_config_file(basename)
    both use existing methods or variables and create a single centralized point to access shng's config dirs and files (no more accessing sh._etc_dir or os.path.join-ing chains, mostly)
    exceptions are sh.get_basedir and sh.get_confdir (for extern_conf_dir) as I see these to be special cases. They could be adjusted quite easily, though
• lib.constants has a bunch of DIR_FOO and BASE_BAR constants designating config dirs and config file basenames. These can (should) be used in calls to the mentioned methods.
• these are also present in the test mockup core
• all lib/.py and module/admin/.py use these methods consistently

Running different setups and tests all went smoothly, threw/logged no errors and functioned as intended, so no obvious errors

All "legacy" methods and vars are still available, so not using the new methods will still function (non-breaking change); we might designate the old methods deprecated at some later time.

I intend to merge this soon; though non-breaking, I offer the PR up for testing on other setups beforehand.