Support Tor Router/Transtor mode in Tor Browser

Not all TBB users actually want to use the TBB-bundled tor and vidalia. Some have their own transtor setups/upstream Tor Routers.

In #5611 (moved), we're trying to create an easy way to launch Tor Browser from the same bundle to support these configurations without shipping different builds. In #5658 (closed), we're thinking about adding a prompt in Vidalia to directly ask users about the presence of Tor Router.

Both approaches should probably use the same environment variable. It looks like we've settled on TOR_TRANSPROXY=1. It will be relatively easy work to adapt Torbutton to check for this var and turn on transparent proxy mode if it is present.