path-lookup: introduce user_search_dirs() (shall replace xdg_user_dirs())

path-lookup: introduce user_search_dirs() (shall replace xdg_user_dirs())

xdg_user_dirs() doesn't seem well-organized currently.
In all other xdg_user_*() funcs we assume /etc/xdg/systemd
to be a symlink to /etc/systemd/, hence it is the odd one out.
Also, when the relevant envvar is unset, it only returns
the global search dirs.

sd_path_lookup() actually covers this nicely with SD_PATH_SEARCH_*,
where the combined search paths (from user home and system) are used.
Therefore, let's introduce a wrapper for that, and deprecate xdg_user_dirs()
(would be removed in later commits).