Bug 7895: Test for PSGI-related env to report in about.pl

Bug 7895: Test for PSGI-related env to report in about.pl

This patch introduces a test for psgi/plack in %ENV and uses that data
to report in the about page.

To test:
- On current master (without Plack), go to about.pl
- Notice no mention of Plack
- Apply the patch
- Reload the about page
=> SUCCESS: No change
- Enable plack, and access the about page through Plack
=> SUCCESS: There is a PSGI label, showing that you are running Plack
- Sign off :-D

Regards

PS: I haven't tested the mod_perl scenario, I just grabbed the MOD_PERL env var
from the docs.

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>