Bug 13791: Plack out-of-the-box support on packages

Bug 13791: Plack out-of-the-box support on packages

This patch introduces a koha-plack script that controls running Plack
processes for each instance. They are run using 'starman', listening
on a Unix Domain Socket (UDS):

  /var/run/koha/<instancename>/plack.sock

The plack configuration file[1] is expected to be on:

  /etc/koha/plack.psgi

and is installed by the package.

It also adds the following helper functions to koha-functions.sh:

 - is_plack_enabled
 - is_plack_running

Done:
- koha-plack script
- suitable psgi file

To test this patches you will need to install
- starman
- libplack-middleware-reverseproxy-perl

[1] Yeah, a single file. Because we will be relying on multiple mount
points for each "app" (i.e. 'opac' and 'intranet', and 'api' ;-) )

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@unc.edu.ar>