Support key prefix by bin

To make memcache session handling easily shared across a multi-site install, we really need to be able to have prefixes defined by bin. My patch attempts to handle this a lot like the DB prefixes are handled. If the prefix value is a string, it's applied to all keys. If it's an array, it uses the appropriate prefix. The 'default' key will be the default prefix just like the DB code does it.

The patch is against 6.x-1.7, but it doesn't look like the dmemcache.inc code in the 6.x branch has really changed since then.