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

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

... turns out it's faster (surprise! ;))

Due to inigrep limitation, we cannot automatically cherry-pick hook.ini
based on where the hook is found--we can only have all hooks in one
file, or use join strategy.

Add section like this to hook.ini:

    [hook.mycomm]
        pre = echo $SATURNIN_SUBCOMMAND will run soon
        pre = echo $SATURNIN_SUBCOMMAND is almost running
        post = echo $SATURNIN_SUBCOMMAND has finished now

"@" breaks the string to several arguments, which is not what we want
here

Config files can now be merged from 4 sources:  distribution-provided,
(share folder), local (/etc/saturnin), user-specific (~/.saturnin and
~/saturnin/ini.d/*)

Paths are transferred by saturnin meta-command so there's no need
to build sub-command binaries

Move cub-command to libexec path to prevent calling them directly,
allowing for better control of environment.