Will make code less ugly at places.

Be nicer to people who get help by trial and error.

The new Shellfu brings some large changes.  Among what touches us
directly:

 *  Most global variables were renamed (removing SHELLFU_* prefix).

 *  inigrep API has been revamped and the file merging/guessing
    feature has been dropped; it now belongs to us.

subcommand (binary) and its arguments are both already is debug-shown
inside saturnin__runsc; cases when that does not get called are normally
not interesting.

Scripts may need it (eg. if using `saturnin__runhook()`) but exporting
it in the middle of main script makes it harder to see the scope
clearly and also is ugly.

Tweak the saturnin__runsc syntax so that it's not dependent on this
being global and export the SATURNIN_SUBCOMMAND only for the actual
subcommand binary call.

Since saturnin's dedication to be "meta-command" builder, the distinction
between "meta" and something else does not make much sense.

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

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.