Swap round some tmux bindings.

Swap round some tmux bindings.

Use h and l to switch left and right in panes; H and L to switch left
and right in windows.
Use J and K to switch up and down not top and bottom which only worked
when there was one pane on top or bottom, ie they broke if there were
two vertically-split panes on top.

These changes diverge from screen compatibility somewhat but
it was getting annoying to have to swap between using shift or not
when changing panes horizontally or vertically.

Also use 6 as another way to switch to the last pane and ^ to switch
to the last window, kinda like Vim.