So that one can still build without having it set properly.

If for certaiun group a mode is not specified, fall back to
a value inhertited from environment, or eventually to 644.

The version based on config.mk has been finalized to include more
features:

 *  Build variables are now defined in config.ini.
 *  The ini system can self-reference other keys (up to several levels).
 *  Some config (namely `ENV` section) can be overrided by environment
    variables.
 *  The installer is now more portable (ported to saturnin so far):
    everything can be defined in config.ini

Make it

 *  more flexible,
 *  more readable,
 *  faster,
 *  take care of manpages (not fully done yet),
 *  take care of dist list,
 *  little bit more polished in various other things.