config: add support for conf.d style
Distros may want to overwrite various parameters of GeoClue's config file without shipping a complete config file which may result into out-of-sync config files when newer GeoClue versions are released. Support /etc/geoclue.d/ as a config directory in which distros can place overwrites, the config files are loaded in alphabetic order and overwrite the original config file parameters stored in /etc/geoclue/geoclue.conf.
This patchset is written on top of !140 (closed) as a new parameter is introduced there as well.
If the meson requirement bump is too big, it can be replaced with a custom script as well, see https://mesonbuild.com/Release-notes-for-0-60-0.html#install_emptydir-function Fine as Ubuntu 22.04 has meson 0.61.2
Implements #89 (closed)
CC: @craftyguy