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

Actually ShellCheck was *right* that the variable expansion needed to
take care of spaces in paths, but the fix (g:b68913c in ffoo repo) was
totally wrong and only broke things.

Lessons learned:

 *  *Do* run ShellCheck et al., but still **DO CAREFULLY** consider
    effects in the particular context.

 *  Too much coffee, repetitive work *and* distraction is a good way
    to ask for trouble.