Implement MySQL local (socket) connection. Fix #39.

The parser was happily parsing such a connection string as the
following, but the rest of the code didn't really know what to do about
it:

  mysql://unix:/var/run/mysqld/mysqld.sock:/main

In passing, fix bugs where the PostgreSQL unix domain socket connection
was still shy of a brick load, omitting to consider the case where the
connection host is actually a list of '(:unix . "path/to/socket").