'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

Sneaky change from using mc as an example to using vifm. It's closer to
ncdu in spirit.

The default shell was changed from /bin/bash to /bin/sh in commit
61a7846c ("config: Use /bin/sh as default shell and don't check for its
existence", 2015-04-05), update the documentation about this.

Check if the environment variable NCDU_SHELL is defined before the SHELL
variable is checked. This makes it possible to specify a program to
execute when 'b' is pressed. Setting SHELL to for example "mc" (Midnight
Commander) didn't work because mc already uses SHELL to execute
commands.

Required for the W* macros on OpenBSD. Reported by Brian Callahan.

Can cause too much confusion otherwise; The imported data may not at all
reflect the filesystem that ncdu has access to.

/bin/bash is not available on every system, nor always the default.
/bin/sh is standardised, but not always the preferred interactive shell,
hence it's configurable.

There's no need to check for the existence of this shell during config,
since it's an option that only affects run-time - and you may compile
ncdu on a totally different system than you would run it on.

The check for the system() exit status is slightly problematic, because
bash returns the status code of the last command it executed. I've set
it to only check for status code 127 now (command not found) in order to
at least provide a message when the $SHELL command can't be found. This
error can still be triggered when executing a nonexistant command within
the shell and then exiting.

Key 'b' in the browse window spawns a shell in the current directoy.

We first check the $SHELL environment variable of the user for the preferred
shell interpreter. If it's not set, we fall back to the compile time
configured default shell (usually /bin/bash).

Signed-off-by: Thomas Jarosch <thomas.jarosch@intra2net.com>

As per http://dev.yorhel.nl/ncdu/bug/55

Slightly modified patch from Gianluigi Tiesi.
http://dev.yorhel.nl/ncdu/bug/47

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