bashrc,setup.sh- Disable tmux in the startup shell

This fixes a bug with tmux autostarting in the login shell and setting
the $TMUX var, causing all child processes to think they were inside
tmux, preventing it from starting normally.

This does not fix the problem of launching a terminal from a terminal -
the tmux variable is not reset.

I think this might be a valid indication that I should set tmux to
launch using the terminal itself, not bashrc, but I don't have an easy
to use text-based configuration file for gnome-terminal, so I can't put
it under version control. Considering that, I will keep this fix until I
run into a problem with it.