Bug 1093025: Initialize listen socket only once in |UnixSocketWatcher::Listen|, r=kyle

Sockets in listen mode may only be created and bound once. This patch fixes
|unixSocketWatcher::Listen| to respect these contraints. Listening more than
once will reuse the settings from the first listen operation.
Bug 1093025: Move |UnixSocketConnector| to separate file, r=kyle

|UnixSocketConnector| will be shared by |UnixSocketConsumer| and
the upcoming listen and stream sockets.
Bug 1093025: Add |ListenSocket|, r=kyle

The new class |ListenSocket| listens on Unix sockets for incoming
connections. Accepted connections are handed over to a compatible
connection-oriented socket.
Bug 1093025: Add |StreamSocket|, r=kyle

|StreamSocket| implements a |ConnectionOrientedSocket| for stream
sockets. It utilizes |ListenSocket| to wait for incoming connection
requests.