Small EON Steel fix in libdivecomputer for very active divers

[Linus Torvalds]

Jef reports that somebody filled up their EON Steel, and then the new
dives would no longer download reliably.

We used to just rely on new dives being last in the readdir() from the
EON Steel filesystem image, but it turns out that that is only true
until the dive list fills up (at about 400 dives). Then it starts
overwriting old entries.

And when that happens, subsurface will start seeing older dives first,
and go "I've already seen this" and stop downloading - without ever
getting to the new dives.

So to fix that, the EON Steel downloader is now sorting the dive list
by date, which has its own failure mode (don't screw up the date on
dives!) but at least doesn't cause problems under normal
circumstances.

This probably doesn't affect a whole lot of people - you need to dive
a lot to fill up your EON Steel since it was released (and the people
who dive *that* much often stop bothering to log their dives), but
hey, it's definitely possible and has happened.

So if somebody has hit this, there's now a fix in the current
Subsurface branch of libdivecomputer.

                  Linus