pin: Fix the template.

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.
pin: caller_module_name is not a topscope variable
unattended-upgrades: Fix origins for Squeeze.

Because Squeeze is now oldstable we need to add an oldstable line too
otherwise security updates won't be picked up. This is still because we
can't match on codename.
Merge pull request #281 from daenney/unattended-upgrades-squeeze-origins

unattended-upgrades: Fix origins for Squeeze.
Merge pull request #280 from daenney/fix-pin-comments

Fix pin comments
Add security warning re: short keys
Merge pull request #282 from laurenrother/security

Add security warning re: short keys