still todo:
  - 'fix' is missing
  - 'del -R' does not work
  - 'upgrade' does not do self-upgrade first

... and a lot of testing.

this allows quite some optimizations to running time and memory
requirements.

 * basic code for a backtracking, forward checking dependency satisfier
 * works better when there are tricky dependencies to solve
   (when can't just upgrade everything to most preferred versions)
 * the new code always evaluates all of 'world' constraints
   (old code just does incremental updates based on heuristics)
 * is probably somewhat slower than old code (probably unnoticeable
   difference in most cases)
 * makes easier to write support for provides and repository pinning
 * test applet and a bunch of test cases added which uses the new code
 * from the old feature set install_if is not yet implemented

This applet can be used to generate data for graphviz tools.
Useful to visualize package dependencies, and possible errors
in the repository.

Usage examples:
  apk dot gnome-desktop | tred | dot -Tpng gnome-desktop.png
    -- to generate simplified dependency chart of gnome-desktop
  apk dot --errors | dot -Tpng index-errors.png
    -- to generate chart visualizing dependency errors in index

We want minimal static build. And this now also breaks with our openssl
since it tries to automatically dlopen some of the engine modules.

Make the libapk and lua module optional at buildtime and off by default.
This is because enabling libapk requires that mkinitfs also copies the
libapk.so or the initramfs will not work.

To build libapk:

 make LIBAPK=yes

To build lua module (will enable LIBAPK=yes):

 make LUAAPK

To link apk to libapk (will enable LIBAPK=yes):

 make SHARED_LIBAPK=yes

All the above applies to make install target as well.

Instead of having a null pointer, use a dummy array which just
says the array is empty. This helps in multiple places of the code
which would otherwise need explicitly need to check first if the
array exists. This has been cause of multiple seg.faults in the
past as the array check is easily omitted.

This also removes (or fixes) all existing checks accordingly.

also makes it possibly to upgrade package without adding it to
top-level deps. fixes #69.

this way we never change cwd, and relative filenames are always
parsed consistently. this also helps filename construction in many
places. this patch also changes '--root' to override location of
all configuration to be in the new root. previously it depended
on the file which one was used.

apparently the double colon rule dependencies affect only the
specific rule-set.

an utility to check package signature and integrity.

use pkgconfig of zlib instead of hard requiring it. and link in
the pkg-config libs with --as-needed as not all of the openssl
libs are really required.

instead of having static md5 implemenation, use the openssl
library for digest functions.

Administrative tool to download or delete files to/from the cache.

Currently just goes through all world dependencies and updates them
where possible (ref #51).

Cache non-local index files always locally.
Introduce 'update' applet to force refresh of cached index files.
Fixes #19.

Tracks now probler header file dependencies and command line parameters
used to build files. E.g. changing CFLAGS rebuild all C-files. And changing
version rebuild now the files where it's used.

Fixes #24.

Currently only implement --backup to get list of (config) files in
protected directories to backup.

This also fixes a database corruption bug in database.c.

breakage and major changes.