To distinguish between different kinds of config/data files, this commit
introduces two new settings, the "config directory" and the "webroot
directory". The split is meant to be implemented like this:
- files written by the dss should go into the 'data' directory
- files written by the users should go into the 'config' directory
- web server files should go into 'webroot' directory

WITH_CONFIG and WITH_WEBROOTDIR default to $WITH_DATADIR{,/webroot}