(cherry picked from commit ce7036d249b6f05633c343ab95d88552aab85402)

(cherry picked from commit 9801f46ece0ca2525f02d71464efc42296dddcb5)

(cherry picked from commit e4e8ebd9e0037812436a1588809deb23e0f3751a)

They were defined (rather than just declared) in .h files, when they
really should be defined in at most a single .c file.

Fixes #140.

Fixes #135

This is a best-effort approach to save ncdu state when memory is low.
There's likely allocation in libraries that isn't being checked
(ncurses, printf).

Fixes #132 (it actually doesn't, that needs a 64bit static binary too,
but I'll get to that)

https://dev.yorhel.nl/ncdu/bug/112

Unfortunately, there wasn't a single bit free in struct dir.flags, so I
had to increase its size to 16 bit. This commit is just the initial
preparation, there's still a few things to do:

- Add "extended information" cli flag to enable/disable this
  functionality.
- Export and import extended information when requested
- Do something with the data.

I also did a few memory measurements on a file list with 12769842 items:

  before this commit:    1.239 GiB
  without extended info: 1.318 GiB
  with extended info:    1.698 GiB

It's surprising what adding a single byte to a struct can do to the
memory usage. :(

I've decided not to use ls-like file name coloring for now, instead just
coloring the difference between a (regular) file and a dir.

Still looking for a good color scheme for light backgrounds.

TODO:
- Add (ls-like) colors to the actual file names
  -> Implement full $LS_COLORS handling or something simple and custom?
- Test on a white/black terminal, and provide an alternate color scheme
  if necessary.
- Make colors opt-in?

The total size isn't calculated when exporting to a file.

2 billion files should be enough for everyone. You probably won't have
enough memory to scan such a filesystem. int is a better choice than
long, as sizeof(int) is 4 on pretty much any system where ncdu runs.

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.

This silences clang and gcc with -O2 -Wall -Wextra

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.