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.
The architecture is explained in dir.h. The reasons for these changes is
two-fold:
- calc.c was too complex, it simply did too many things. 399ccdeb is a
nice example of that: Should have been an easy fix, but it introduced
a segfault (fixed in 0b49021a), and added a small memory leak.
- This architecture features a pluggable input/output system, which
should make a file export/import feature relatively simple.
The current commit does not feature any user interface, so there's no
feedback yet when scanning a directory. I'll get to that in a bit.
I've also not tested the new scanning code very well yet, so I might
have introduced some bugs.
