There are 2 reasons why you would want to share cache-blocks across siteaccesses that are for instance only different in design (css):
1. Save space, cache-block data can become quite huge ( seen 101Gb on a medium site recently with 6 siteaccess that only differ in css )
2. Performance, bigger probability to get a cache hit.

This patch adds a new attribute on cache-block called "ignore_access_name" that if present makes the cache-block NOT use the siteaccess name as part of the cache key.

NB: users need to add language code as cache key if they use this on multilingual site.
NB2: users should not use this if cache-block surrounds links and site is not in vhost mode (read: siteacces is part of relative urls)

Bonus question: Why is $subtreeExpiry defined twice as input parameter for eZTemplateCacheFunction::processCached?