configure.in -> configure.ac

As recent autoconf wants
util.c: Fix possible buffer overflow in fillsize() and formatsize()

They should now be able to cope with file sizes in the full (positive)
range of a signed integer, i.e. 0 bytes to 8 EiB minus one byte. The
size calculation of directories, however, may still overflow and cause
negative integers to be passed around. That should be fixed.
Clip directory sizes to fit in a signed 64bit integer

This mostly avoids the issue of getting negative sizes. It's still
possible to get a negative size after refresh or deletion, I'll get to
that in a bit.
Clip directory sizes to make sure they are positive

I realized that I used addparentstats() with negative values when
removing stuff, so it had to be done this way (without rewriting
everything). It's a simple solution, anyway.
gitignore
'c' to show child item counts
'C' sorts by items
use the locale's thousand seperator, if available
ncdu.1: Add new c/C keys
Use pkg-config to detect ncurses

Ncurses provides a pkg-config module which could be used here. If not
available we fall back to the old detection heuristic.

Signed-off-by: Justin Lecher <jlec@gentoo.org>
configure: Use PKG_PROG_PKG_CONFIG + mention pkg-config dep in README
Add CACHEDIR.TAG support.

A new command line parameter allows to filter out directories containing
the proper `CACHEDIR.TAG` file.
See http://www.brynosaurus.com/cachedir/
CACHEDIR.TAG - improve code style and the name of the parameter.

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)
Let has_cachedir_tag use `malloc` instead of a buffer on the stack.
Document `--exclude-caches` in ncdu.pod.
has_cachedir_tag: Keep statuc allocated buffer for paths.
Use /**/ comments + minor style change
dir_scan.c: Reset directory sizes when excluded by a CACHEDIR.TAG

Tiny bug fix: The size of an excluded directory entry itself should not
be counted, either. This is consistent with what you'd expect: A cache
directory with thousands of files can easily take up several megabytes
for the dir entry - but from the perspective of a backup system that
recognizes cache dirs - the dir is empty, and therefore shouldn't take
any extra space at all.
Move khash & yopt to deps/

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.
deps: Update khash and yopt to their latest upstream versions