ini_file: Sort [default] sections before others.

It is common (but not universal) for INI-style files to have a section
named [default], or [DEFAULT], which holds defaults for all other
sections.  For example, the 'configparser' Python module implements
that behavious.

While the [default] section typically doesn't *have* to be placed
before the sections it provides defaults for, it tends to make the
file easier to read if it *is* placed first in the file.

We thus here make sure to put all sections named "default" (case-
insensitive) first in the generated file.