(cherry picked from commit a076ac714aa6fb24e0e9eb227372df84a4b6000b)

(cherry picked from commit ad5b7fce74a3b7b0ed726620c81ea552c643cdad)

(cherry picked from commit e4e8ebd9e0037812436a1588809deb23e0f3751a)

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

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

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.

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.

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

Turns out that being able to open an empty directory actually has its
uses:
- If you delete the last file in a directory, you now won't be directed
  to the parent directory anymore. This allows keeping 'd' pressed
  without worrying that you'll delete stuff outside of the current dir.
  (This is the primary motivation for doing this)
- You can now scan and later refresh an empty directory, as suggested by
  #2 in http://dev.yorhel.nl/ncdu/bug/15

Some measurements importing a gzip-compressed file (zcat .. | ncdu -f -)
containing a bit under 6 million items and a few choices of how often to
call input_handle():

Called on every item:

  real    0m13.745s
  user    0m12.576s
  sys     0m4.566s

Called on every 8 items:

  real    0m7.932s
  user    0m9.636s
  sys     0m1.623s

Called on every 16 items:

  real    0m7.559s
  user    0m9.553s
  sys     0m1.323s

Called on every 32 items:

  real    0m7.279s
  user    0m9.353s
  sys     0m1.277s

Called on every 64 items:

  real    0m7.166s
  user    0m9.389s
  sys     0m1.117s

Called on every 256 items:

  real    0m7.073s
  user    0m9.439s
  sys     0m1.027s

32 seemed like a good compromise.

- errors in item() didn't propagate properly
- empty [] and {} values weren't allowed
- fractional numbers weren't allowed
- parsing of escaped characters didn't ensure that enough data was in
  the buffer
- E() didn't propagate errors properly in all cases

I'll do some more testing later on, but the current code seems to be
working quite well already.

This is the first working version. There's a few TODO's left.