Upon Aaron and Bernard request this is a very simple single pass caching

Upon Aaron and Bernard request this is a very simple single pass caching
of the mailbox list. It's not a big deal as only very specific tasks calls
the mailbox list twice (purge for example).

Wouter please check if this enters into your plans and roll it back if not
8) The caching is quite simple and shouldn't have collateral effects. The
caching is only active during the page refresh as it is not sessionized
(and shouldn't as third party software could change mailbox).

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@2471 7612ce4b-ef26-0410-bec9-ea0150e637f0