Bug 17262: Inject MEMCACHED_* variables into Plack's env

Bug 17262: Inject MEMCACHED_* variables into Plack's env

This patch adds two functions to koha-functions.sh to read the
MEMCACHED_* variables from configured instances' apache files (the only
source for that info for now).

It then tweaks koha-plack's start_plack() function so it exports the
variables to env before launching starman.

The patch seems trivial, but it highlights a bigger problem with our
Plack integration as the starman server seems to hang if those
environment variables are defined!

Sponsored-by: ByWater Solutions
Signed-off-by: Frédéric Demians <f.demians@tamil.fr>
(cherry picked from commit 29df6161ef24e29984d5379640faad24d0e73161)
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>