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>

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

This is a slightly modified patch contributed at

  http://dev.yorhel.nl/ncdu/bug/35

Both microlibraries come from another source, and are not maintained as
part of ncdu. This separation of src/ and deps/ makes the relation more
clear.

Use a macro instead of the global constant `cachedir_tag_signature`.
Use `memcmp` instead of `strncmp`.
Add `has_cachedir_tag` to exclude.h.

(See http://dev.yorhel.nl/ncdu/bug/30)

A new command line parameter allows to filter out directories containing
the proper `CACHEDIR.TAG` file.
See http://www.brynosaurus.com/cachedir/

This also adds the possibility to combine short options that expect an
argument, e.g. "-xo <file>" or even "-xo<file>".

I've no idea for what other purpose I'd use numbered options. This is
much shorter, and can easily be combined with other options (e.g. -rx1).

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

!WARNING! The export option is experimental, and the file format is not
final. I make no promise that a future version of ncdu will be able to
read the current format. There's also quite a few TODO's left.

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.

This fixes a bug where ncdu would stop scanning a directory if the
terminal window has been resized to a small enough space that the
warning would show up.

Damn, it's 2012 already.

This should also allow compilers to generate better code.

This (in combination with linking to ncursesw) fixes the display of
non-ASCII characters.

There shouldn't be a need for such a state when there's a central
place where the program execution keeps returning to.

So we're actually back to having one header file for everything,
except it's now maintainable.

Looks much better this way!

And with that, finally managed to get rid of sflags in ncdu.h

Another code cleanup, many more to come...

Because that's where it's supposed to be.
(geez, the current code is still a mess)