Fixes #135

Broken in 77aca35f
Fixes #134

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)

Fixes #128.

This allocation is currently leaked, but as long as we don't allocate
new ones for each refresh, that shouldn't be much of an issue.

(cherry picked from commit 9dc2d32a8fb557112f8f96cd14d50b934543ce39)

stat_to_dir() assumes that buf_dir is clean; calling it twice breaks
that asumption.

Symlink loops are handled by the stat(2) syscall. Symlinks pointing to a
directory are ignored (to avoid loops in the recursive scan).

This adds an 'm' command to show the latest modified time of all files
in a directory. The 'M' command allows for ascending and descending
mtime sorting. These are only enabled with the -e flag and overload
the dir_ext mtime field.

I had taken care to not sort empty directories during dirlist_open(),
but forgot that manual user actions can still cause dirlist_set_sort()
to be called, which does not handle empty directories.

Reported by Alex Wilson.

https://dev.yorhel.nl/ncdu/bug/125

Based on https://dev.yorhel.nl/ncdu/bug/124

Implements https://dev.yorhel.nl/ncdu/bug/43

Ought to fix https://dev.yorhel.nl/ncdu/bug/115

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

https://dev.yorhel.nl/ncdu/bug/111

'char' may be unsigned on some architectures, which will cause the
"overflow check" on decrement to fail.

This would at most result in a confusing UI issue where no confirmation
option appears to be selected.

It's looking a bit cramped, but I'm lazy.

Fixes a minor glitch where A_REVERSE would be set on the information
window if the last item in the dir is selected.

And stick to the more portable second resolution timestamps for mtime.

And document the --color flag that I forgot.

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. :(

Fixes https://dev.yorhel.nl/ncdu/bug/103

I don't think a stack overflow as a result of recursion is exploitable
on a modern system. It should just result in an unfortunate write to a
page that is not writable, followed by a crash.

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.

This should fix https://dev.yorhel.nl/ncdu/bug/99 - with the downside
that this requires a C99 compiler.

I also replaced all occurrences of static allocation of struct dir with
use dynamic allocation, because I wasn't really sure if static
allocation of flexible structs is allowed. In the case of dirlist.c the
dynamic allocation is likely required anyway, because it does store a
few bytes in the name field.

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?

Fixes https://dev.yorhel.nl/ncdu/bug/95

Fixes https://dev.yorhel.nl/ncdu/bug/94