Add support for MySQL sockets

On my server, connection to MySQL is via a UNIX socket - not via networking.

See the MySQL "skip-networking" configuration option: http://dev.mysql.com/doc/refman/5.6/en/server-options.html#option_mysqld_skip-networking

Currently the Mysqli.php driver only supports network connections.

It is conventional to interpret a host-name beginning with a "/" as a path to the socket file instead of a hostname.  e.g.  "/var/run/mysqld/mysqld.sock"