Improve start-tor-browser argument handling on Linux
We should allow more than one argument to be specified at the same time, and generally behave better when this happens (right now, we just pass those extra args to Firefox, which is not great).
I was going to do this after 4.5-stable, but weasel has a number of related complaints about our argument handling, so I might as well fix all of this in one go. I will add those as child tickets.