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.