Make them shorter but also sortable (ie. 'conv' for converters, 'flt'
for filters.)

Allow overriding each of the essential parts at CLI, in which case do
not load them from INI.

New command allows to define flexible menus in INI files.

Single INI root now contains all menu definitons.  Each menu consists of
item generator, selector and consumer, which are checked and chained
together.

Alternatively to specifying a generator, items can be listed right in
the INI section.

Sub-commands in libexec must use saturnin_wraphook if they want pre/post
hook, or may use saturnin_runhook to run their own hooks as they wish.

This module is intended to be moved around with saturnin, but unlike
main meta-command (bin/saturnin.skel), it should not be necessary to
rename it or edit to adapt to new project.

Add checks for syntax errors and missing SATURNIN_SUBCOMMAND.  In both
cases, ignore the hook.

Minimize the "built part" and code repetition among saturnin "clones".

To allow for experimenting with functions from embedded library.

This should help test and manage embedded Fastfoo library

mkusage exits by default so it's intended to exist as one call per
process only.

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

 *  Add docstring,
 *  add explanations for local variable names,
 *  separate export declarations,
 *  sort globals alphabetically,
 *  add built part delimiters,
 *  quote paths properly.