Bug#102423: no errors on invalid syntax in /etc/apt/preferences

[Wichert Akkerman <wichert@wiggy.net>]

reopen 102423

Previously Jason Gunthorpe wrote:
> As I said, It isn't incorrect, it is an unsupported field, just like any
> other file in that format unsupported fields are ignored. This is so if
> you have to downgrade apt it doesn't break parsing the preferences file.

So make it print a warning that it encountered an unsupported field.
That way you keep downwards compatability while also notifying the
administrator of possible brokeness.

Wichert.

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