Automatically remove packages that are no longer required by a top
level dependency.

Refuse to delete explicitly specified top-level packages unless
--force is specified.

Enforce name_id to be within apk_state allocated area. New apk_name:s
can be created later for e.g. unknown packages requested at command line.

Fixes #24.

Calculate changesets directly by stabilizating the package graph instead of
recalculating the whole graph and then diffing (similar approach as seen
in 'smart' package manager). The algorithm is not complete: defferred
search space forking is missing. So you don't always get a solution on
complex graphs.

Benefits:
- usually the search state tree is smaller (less memory used)
- speed relational to changeset size, not database size (usually faster)
- touch only packages related to users request (can work on partitially
  broken state; upgrades only necessary packages, fixes #7)

Also implemented:
- command prompt to confirm operation if packages are deleted or downgraded
- requesting deletion of package suggests removal of all packages depending
  on the package being removed (you'll get list of packages that also get
  removed if you want package X removed)
- option --simulate to see what would have been done (mainly for testing)
- an untested implementation of versioned dependencies and conflicts

A lot has changed, so expect new bugs too.

This will be used later by the commit change calculator (for improved
changeset calculation, ref #7). Will be also used by "apk info" to show
reverse dependencies or "required by" information.

fixes issue with apk version -t asdf asdf

This is so we can handle arpwatch-2.1a15
This makes us not 100% compatible with Gentoo

Otherwise we only allow letters after first digit, i.e 2a, 2b but not
2.2a, 2.2b etc.

speeds up writing of package / file database.

So we don't get artificial limits on the amount of dependencies
(fixes #8).

So you will not get .apk-new files of the new configuration files.

we let bb use /bin and /sbin, all packages overriding busybox must
install the bins in /usr/bin and /usr/sbin

- empty lines comes after not before
- separate pkgname with : when using -v option