tmux: start pane numbering at 1
tmux: start pane numbering at 1

We already do this for windows; do it for panes as well, and for the
same reason (0 is too hard to reach).

wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent