Allow Ctrl-b to work in nested tmux

[ccaominh]

When starting a nested tmux, do not revert the tmux prefix in the parent tmux to the default (C-b) as it intercepts Ctrl-b from the nested tmux. Instead, bind the tmux prefix to C-F12, which is unlikely to be used and parallels the M-F12 binding used for active handler.

For example, to reproduce the problem:

1. Start tmux
2. Start vim in paraent tmux
   • Ctrl-f works to go forward a page
   • Ctrl-b works to go back a page
3. Create nested tmux (C-a C-s)
4. Start vim in nested tmux
   • Ctrl-f works to go forward a page
   • Ctrl-b does not work to go back a page