This mostly avoids the issue of getting negative sizes. It's still
possible to get a negative size after refresh or deletion, I'll get to
that in a bit.
This allows scanning stuff without initializing ncurses. Not too useful
at this point since ncdu will switch to an ncurses environment when it's
done anyway, but this will become more useful when the export-to-file
feature has been implemented.
POD is somewhat more simple and flexible. I now use ncdu.pod to generate
a nicely formatted manual page on the ncdu homepage, rather than
displaying a rendering of ncdu.1 formatted in a monospace font.
The tarball will still contain an ncdu.1, so there's no extra dependency
on pod2man. (Unless you clone from git, since ncdu.1 isn't in the repo)
