Some python tweaks to speed glsa-check

svn path=/trunk/gentoolkit/; revision=646

http://git.overlays.gentoo.org/gitweb/?p=proj/gentoolkit.git;a=commit;h=10e55d71bd5914fc7c9082adadf7bde2bec14ae3
Change behaviour of getMinUpgrade

This allows to differentiate between situations where
the system is unaffected and unexistance of an upgrade path.

Previously, the glsa-check would treat GLSAs that had no
upgrade path (such as mask glsas) as not affecting the system.

svn path=/trunk/gentoolkit/; revision=647

http://git.overlays.gentoo.org/gitweb/?p=proj/gentoolkit.git;a=commit;h=ef38a394c5c2f5901173a53914705730850f9b3f