Ignore compiled files.

Ignore compiled files.

Add new feature 'preserve-libs' that injects files into $D that
- belong to a package instance that is about to be autocleaned
- are not installed by the new package instance
- are listed in at least one NEEDED file in vdb
- are not just used by other files of the package instance
As a side-effect a new file /var/db/pkg/.NEEDED is created that
contains a reversed map of the NEEDED files found in a packages dbdir.

svn path=/main/trunk/; revision=5975