lib/package_unpack.c: fix a memleak.
New code to find pkg obsolete files, with a proper kyua test.

Previous code incorrectly found obsolete files in this case:

(curpkg) - in files: /etc/foo.conf
(newpkg) - in conf_files: /etc/foo.conf

and removed this file because the code couldn't find it in the same
array. The new code fixes this case and compares the whole pkg filelist.