tmux 1.8 changed how run-shell works.

tmux 1.8 changed how run-shell works.

Prior to tmux 1.8 all run-shell commands ran in the foreground and we
had to detach explicitly.  From 1.8 onwards we can use the -b flag to
run a command in the background.

Ensure we run ktmux_helper in the appropriate way for the available tmux
version.