Support for listening on UNIX sockets

[vlasky]

This specifies a path to a UNIX socket file to listen on instead of the TCP port.

If specified, SQLPage will accept HTTP connections only on this socket and not on any TCP port. This option is mutually exclusive with the listen_on and port options.

Useful when running SQLpage behind a proxy server like Nginx, as the overhead for communication using Unix Domain Sockets is less than when using the TCP stack.