Don't allow to plan dives out of order

The planned dive always has to be the last dive in the dive list. To make
sure of that we interpret the relative start time to be relative to either
the current time or the end of the last dive, whichever is later.

This fixes a bug where we would delete the wrong dive and get our data
structures confused by planning multiple dives out of order.

Reported-by: Sergey Starosek <sergey.starosek@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>