Enable socket connections

I'd love to see Memcache API support connections via unix sockets out of the box. As I as I can tell omitting the tcp overhead and intead using socket connection one can improve memcache performance by up to 10%. (Of course, only if respective memcached daemon runs locally).

Please consider attached patch. I am running a site with said modification and have not been running into any problems so far (using memcache.db.inc).