On non-Windows platforms, use a Unix domain socket by default for the
Tor control port. New preferences:
  extensions.torlauncher.control_port_use_socket (Boolean)
  extensions.torlauncher.control_socket_path (path string)
New environment variable: TOR_CONTROL_SOCKET

When starting tor, always include a ControlPort argument so that
users can use environment variables to switch between the two control
port types.

Add a public TorGetControlSocketFile() function that Torbutton will
use to retrieve the socket file (an nsIFile) when a Unix domain
socket is in use.

Moved the getTorFile() function to our utilities module.