[imp #49] Distribution packaging improved

We will want to package \BlueLaTeX with several boot methods:
 - systemd,
 - jsvc,
 - ... (launchd?)

To allow for this, we want to make packaging configurable, which is
done in the `Distrib` trait of the build.

Distribution is packed with installation script, start/stop scripts, and
is installed by default in /opt.

Close #49 and #100
Merge pull request #122 from satabin/wip/distribution-packaging

[imp #49] Distribution packaging improved