aptitude authentication handling differs from apt-get ones

[Michael Vogt]

aptitude does refuse to trust a package if there is a single non-authenticated
source for the package

apt-get will trust a package if it can get it from at least one trusted sources.

apt-get is correct here because internally libapt will never fall back to a
unauthenticated source when it's downloading some authentiacted.

This difference in aptitude causes install failures in edubuntu (debugged that
with ogra) when base-config run a unsuccessfull apt-get update in stage2 (no
*.gpg files anymore in the lists directory) but authenticated packages are still
available on the cd.

[Michael Vogt (mvo)]

This is fixed in the latest breezy verison