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>