Start using 'plot_info' more for dive-time limits

.. I'll want to move pressure limit calculations into the 'plot_info',
so that we can do several passes of analysis and change dive limits etc
without having to actually modify the dive data itself (or add new
fields to 'struct dive' just for plotting).

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>