Simplify maintenance by moving list of valid_targets to a separate
function.  Both router and usage message are now consumers thereof.

This also changes format of the usage message.

Overview of changes:

 *  Move all project meta-data to mkit.ini (obsolete config.mk)
 *  Add tagline to common peoject meta-data
 *  Use more qualified tokens for replacement
 *  Improve mini-changelog

Make it clear what's related to the packaged project.

 *  Fix tag format errors
 *  Fix VERSION parsing from config.mk
 *  Fix sed error when generating changelog data

v0.0.0 vs vv0.0.0 vs 0.0.0

In local install, the files could be present, so relying on find to find
nothing did not work.  chmod must be properly sandboxed.

Hopefully this is now according to GNU standards.

It looks .dirty as we had to update in 2 steps due to a bug in v0.0.5

Turns out that the old version has always shown the last tag available
instead of last tag available *so far*, i.e. up to the current HEAD.

This made it impossible to determine version correctly when an older
version was checked out, i.e. installing or packaging an older version
would result in broken versioning.

New "functional-like" design makes it simpler to understand, extend,
re-order and re-use blocking conditions.

Also, the "old looks newer" condition was dropped as it's not really
necessary under such simplistic conditions.

Add flexiblility for more specialized targets

Turns out that when DESTDIR is used, the PREFIX dir almost never exists
(such as if DESTDIR points to a build root).

Before DESTDIR support was added, mkit would rely on PREFIX itself.
Now, checks will be done on DESTDIR itself, which must be set internally
in mkit.ini or inherited.  PREFIX is actually considered only inside
mkit.ini itself.

DESTDIR[1] can be now set to specify the root in situations such as when
building a package or installing to a chrooted environment.

If not specified by environment variable (eg. by make) or in mkit.ini,
the default value is governed by MKIT_DEFAULT_DESTDIR, which defaults to
"/".

  [1]: https://www.gnu.org/prep/standards/html_node/DESTDIR.html