Generate date string for the dive list dynamically

.. and sort based on the 'time_t' value itself.

This allows us to use a more compact date format that doesn't need to
sort alphabetically, because sorting by date is always based on the date
value.  So we can use just a two-digit year, and skip the seconds, to
keep the column narrow, while still sorting correctly.

Also, "Depth" is a nice header string, but it is wider than the column
itself, which makes the whole column wider than necessary.  So put the
units in the header instead of in the string, keeping things narrow.

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