(Version): Implement using apt_pkg if APT is available.
(version_compare): Add.

* lib/python/security_db.py
(DB._initFunctions): Directly invoke debian_support.version_compare.


git-svn-id: svn+ssh://svn.debian.org/svn/secure-testing@4236 e39458fd-73e7-0310-bf30-c45bca0a0e42