Overview of changes:
* Added --saturnin-get-subcommands devel key
In last update I inanely removed _ls_sc sub-command used by
completion example in saturnin-demo. This is a final replacement,
which is actually consistent with the rest of plumbing commands.
* Improved docstrings for sfdoc
Added and fixed some old docstrings as well as added module
docstring.
* Improved usage errors with newer Shellfu's pretty.sh interface
pretty.sh in newer Shellfu can handle usage messages in more
user-friendly way, by providing option to append error reason
to the message itself.
(With previous version, one could achieve similar effect but with
more negative effect on readability.)
* Fixed build-time test for Debian
Turns out that dehbelper does not install the package on its own so
we have to deal with it manually.
* Minor code cleanup