Bug 19546: Run starman from the instance's home dir

Bug 19546: Run starman from the instance's home dir

This patch makes koha-plack jump into the instance's home directory to
run.

It is required because Perl 5.18 introduced a breaking change that
makes perl die if @INC includes directories for which the user doesn't
have read permission, and need to be traversed when querying for a lib.
This is the case of '.', which is introduced automatically into @INC
until Perl 5.26 (which removes the 'feature').

The Mojolicious::Plugins lib prefixes the plugin names with
'Mojolicious::Plugin' so it first looks (for example) for
Mojolicious::Plugin::Koha::REST::Plugin::Pagination (Pagination is just
the first one on the list). When it looks for it at '.' it dies (because
of Perl's behaviour) so it doesn't query for the right namespace (the
following to try).

I only reproduced it in koha-testing-docker. To test, you just need to
try this patched koha-plack and make sure it doesn't break due to this
permissions issue.

To test:
- In your koha-testing-docker clone, run:
  $ docker-compose up -p test
- Open a shell inside the container (on a separate terminal):
  $ docker exec -it test_koha_1 bash
- From within the /root directory, restart plack:
  $ cd /root
  $ koha-plack --restart kohadev
=> FAIL: Logs show plack is broken due to permissions problems trying to
    find Mojolicious::Plugin::Koha::REST::Plugin::Pagination
- Use the patched script from this patch:
  $ /kohadevbox/koha/debian/scripts/koha-plack --restart kohadev
=> SUCCESS: Plack runs fine, no error in the logs
- Sign off :-D

Note: people who has environments in which the problems is reproducible,
    please test this version of koha-plack and stamp your sign-off,
    PLEASE.

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>