pkg from only trusted sources keeps being trusted

pkg from only trusted sources keeps being trusted

--allow-unauthenticated switches the download to a pre-0.6 system in
which a package can come from any source, rather than that trusted
packages can only come from trusted sources.

To allow this the flag used to set all packages as untrusted, which is a
bit much, so we check now if the package can be acquired via an
untrusted source and only if this is the case set it as untrusted.

As APT nowadays supports setting sources as trusted via a flag in the
sources.list this mode shouldn't be used that much anymore though.

[Note that this is not the patch from the BTS]

Closes: 617690