planning: Add option to have last stop at 6m/20ft

When diving in areas where there are risk of boats passing above you,
its common practise to do the last stop at 6m to better stay out of
harms way. When doing o2-deco, it doesn't matter for the deco time if
you are doing all the time at 6m, due to that you don't have any inert
gas in your breathing gas.

This code is a reintroduction of 0b8462bd lost somehow between
a70a8898..8fae0031

Signed-off-by: Anton Lundin <glance@acc.umu.se>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>