Handle UNIX domain sockets correctly in memcache & memcached extensions

* Memcache::addServer checks for 'unix://' to determine if the host argument
  represents is a UNIX domain socket. HHVM's version checked for a leading '/'.
* Memcached::addServer checks for a leading '/' to determine if the host
  argument is a UNIX domain socket. HHVM's version did not handle UNIX domain
  sockets at all.

This patch adds UNIX domain socket support to Memcached::addServer, and it
corrects the handling of the host argument in Memcache::addServer.

Fixes #1375.