wlheadless: Set sensible defaults for XDG dirs
The specification says:
| $XDG_DATA_HOME defines the base directory relative to which user-
| specific data files should be stored. If $XDG_DATA_HOME is either
| not set or empty, a default equal to $HOME/.local/share should be
| used.
And:
| If $XDG_DATA_DIRS is either not set or empty, a value equal to
| /usr/local/share/:/usr/share/ should be used.
Set the default values according to the specifications, so that the configuration file can be found even if either or those environment variables is not set.
See-also: https://specifications.freedesktop.org/basedir-spec/latest/index.html#variables