Move satcmd to other repo and distribute saturnin as a stand-alone
shellfu library.  Users can import just the library instead of
embedding it inside their source tree, which enables them to delegate
part of the job to package manager.

We're scratching right ear with left hand...

Absence of quotes breaks queries with spaces.  We're already taking care
of files (implicit or given on command line).

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.