Fix dc nickname helper function

This helper function is used to get a nickname for a dive computer, when
the model information on its own may be ambiguous (ie there may be
multiple dive computers of the same model, and we've nicknamed them by
owner).

However, the helper did completely the wrong thing if it didn't find a
dive computer entry at all due to a missing device ID - it would just
return empty.  Which is bogus: it should return the model name, the same
way it does if the nickname is missing.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Correctly place the first gas label in drop to bottom mode

When dropping to the bottom in plan (and add) mode, the gas label was
placed along the diagonal line from (0,0) to the second dive data point
(i.e. the one at the end of the "at deptch" segment). That looks terrible,
the label needs to be along the segment that we are spending at the
bottom.

This patch fixes that problem.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>