*  dmenu really needs to have the arguments split,
 *  and uris use cat to enable support for - as STDIN

Workspace-based choice was not really that useful, so removing to get
rid of jq dependency

Old code had problems with trailing space--which did not manifest itself
until now when I tried to use infix query (e.g. 'foo%sbar' would always
result in space after '%s').

Turns out that `read` with multiple variables splits the line just as we
need it with no extra leading/trailing junk.

If user does not provide mandatory parameter, `shift 2` will fail but
will not shift anything, resulting in infinite loop.

The routines are all basically related to the meta-command functions.

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".