In APT preferences files the only allowed comments are lines that start
with `Explanation:`, commented lines that start with a # trigger a
myriad of interesting bugs. This is considered a feature of APT.
Because we're only ever writing a single file at a time with only a #
comment at the top we were getting away with this but it shouldn't be
there in the first place.
The two forms of APT preferences records (general & specific) can now be
completely and not partially defined. All distribution properties can be passed
as resource parameters. This change is totally backward-compatible.
