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

We still want user to create PREFIX dir themselves, but the message was
unclear about what is actually wrong.

The existence test is done on next line, and with -f in place,
canonicalization or non-existent PREFIX leads to empty PREFIX which
makes the next message confusing

Use MKIT_INI to specify path or default to mkit.ini in $PWD

 *  Move targets to mkit/mkit.mk,
 *  make `mkit/make` and rest of mkit use MKIT_DIR for own path.

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

Reserve PROJNAME for a "human-readable" name.

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.