Bug 20111: use Unix domain sockets for SOCKS port by default

Retrieve SOCKS port configuration from Tor Launcher if available.
Support the TOR_SOCKS_IPC_PATH environment variable.

Consistently use IPC to refer to Unix domain sockets.

Enhance torbutton_local_tor_check() to correctly handle spaces and
escaped characters within the 'GETINFO net/listeners/socks' response.

Within startup-observer.js, use Services.jsm, Cc and similar
constants, and remove unused code.