config: Add support for XDG config

config: Add support for XDG config

Over the last years, the freedesktop organization has promoted the use
of XDG_HOME_CONFIG for storing configuration data.

Since TVHeadend also wants to help declutter the home directory, lets
follow the spec for our own configuration. While here, reduce the path
from `hts/tvheaded` to just `hts`.

The implementation does not change behavior of existing installations!
If `.hts/tvheadend` exists, it will be used as before.

New installations, will try to use `.config/hts` instead, and
'migrating' is as easy as `mv .hts/tvheadend .config/hts`.

Signed-off-by: Olliver Schinagl <oliver@schinagl.nl>