A less intrusive replacement for _ls_sc subcommand

Overview of changes:

 *  Fixed many packaging errors

    Added proper (build) dependencies, fixed build-time tests (rpm and
    deb), updated meta-data.

 *  Updated code to work with for Shellfu v0.10.0

 *  Removed obsolete internal subcommands

 *  Added support for -V for version (SemVer-formatted)

 *  Changed license to LGPLv2

Overview of changes:

 *  Added option to enable stronger debugging

    You can use PRETTY_DEBUG_EXCLUDE to mute some output from your apps
    that might not be always relevant (example launcher in saturnin-demo
    does this).  This option re-sets the variable to empty string, making
    sure your pretty apps will show as much as possible.

 *  Added tests to deb and rpm packages

 *  Added test header (requires Shellfu v0.9.7+)

 *  Fixed, tidied up and updated project and packaging meta-data

 *  Fixed usage test

 *  Codebase maintenance

$0 is not changed by functions so this could not have worked (strange it
*did* work on Fedora, I still have to find out why).

Overview of changes:

 *  Enabled adding meta command help text

    User can now specify SATURNIN_META_HELP environment variable to
    control what happens when meta-command is called with `--help`
    argument.

 *  Removed `conf` from default sub-command list

    The command is essentially internal; usage from outside is possible
    but requires knowledge normally only available to developers.

New variable SATURNIN_META_HELP can be used to control what happens when
user calls the meta command with `--help` argument.

conf is Saturnin's internal command to access common configuration.  It
could be, in theory, useful for end-user to investigate the configuration,
but they would already need to know how it works and which configuration
keypaths or sections they want to look at.

Bettter hide it; it's not worth for the potential confusion.

Note that the command could actually even be removed one day since
internally it's much more efficient to use saturnin__conf().

Overview of changes:

 *  Properly filter subcommands on listing

    In contexts like main --help and tab completion, Saturnin would show
    also arbitrary files from libexec that did not start with dedicated
    prefix ($SATURNIN_LIBEXEC_PREFIX).  This was a mistake and was never
    intended to work that way.

 *  Hook code is now run as separate bash process

    Running it within sub-command code was a little bit too unsafe;
    there's no need to be evil^Weval here.

 *  Rewrote saturnin__version() (used for app --version)

    Newly, --version will only incorporate tagline if it was defined in
    mkit.ini.

    Also "Powered by Saturnin" line has been added so that we don't have
    to ask user every time.

 *  Added --saturnin-get-saturnin-version

    It's a good idea to include this with test reports and the like so
    we're making it more accessible to scripts.

 *  API documentation clarifications

 *  Codebase maintenance

 *  Refactoring for code readability

Only print tagline if defined (similar to codename); do not make it up
with silly string.

Also we can do away without any local variables *and* very safely inside
79 character width recommendation.

eval was little bit too much.

This was always intended; sub-commands starting with dedicated prefix
is the mandatory convention.

As recommended by Shaturnin style guide:

    https://github.com/AloisMahdal/shellfu/blob/master/notes/style.md

As recommended by Shellfu style:

    https://github.com/AloisMahdal/shellfu/blob/master/notes/style.md

Overview of changes:

 *  Added generic config file finder

    This can be used to find config files other than INIs but using the
    same merging mechanism.  For example, `saturnin__conf_find "foo"`
    should output to first existing instance of file "foo", which could
    live under any of paths set in SATURNIN_CONF_PATH (typically
    /usr/share/app/ini.d/dist/foo, /etc/app/foo, ~/.config/app/foo,
    ~/.config/app/ini.d/my/foo...).

    This function is now also internally usef by saturnin__conf().

 *  Added module-version meta-data for future Shellfu generations

    Current version of Shellfu does not support this yet, but in future
    it should read this and display in lists, man pages, etc.

This should be useful for cases when a sub-command wants to maintain
non-INI config locations.

Overview of changes:

 *  Added saturnin__bug() to help developers help users report bugs
 *  Tweak debug output regarding some globals
 *  Fix document packaging 'doc'
 *  Infrastructure maintenance

The related functionality is stable already and the path is easy to find
out using package manager.