Merge all files:group sections into one, where key is name of the group
and value is source name, optionally followed by destination path.

This breaks old mkit.ini's, but significantly simplifies future ones.

 1. Only one section is needed,

 2. group list is implied from the files section, removing the need
    of lists:group,

 3. destination path has been made optional (by default, basename of
    the source path is used).

Fix up the path joining logic:

 *  For obvious reasons, we panic if group's root is empty.

 *  DESTDIR can be empty, that's normal for installs such as when MKit
    installs its own dogfood.

 *  Root (which sometimes starts with PREFIX) may or may not start with
    slash; relative root such as that is also OK for installs like MKit
    or TFKit.

 *  Furthermore, DESTDIR may or may not end with slash; we have to address
    that and avoid joining the names.

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.

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