Add mysql auth via unix_socket plugin.

Mysql have plugin called 'unix_socket'. It allows authenticate to mysql without password if system user name equal to database user name.
To enable it you need to add to my.cnf:

plugin-load=unix_socket=auth_socket.so

And grant privileges like this:
GRANT ALL PRIVILEGES ON *.* TO 'root'@'localhost' identified via unix_socket with GRANT OPTION;

This patch add parameter 'unix_socket'. If it's True and `allow_passwordles` is also True - unix_socket auth is set for mysql user. Works only if Host='localhost'.