Automatically removing kernels in apt-get could be unexpected, so limit
it to apt for now. To handle --no-auto-remove correctly, rewrite the
hack that makes apt ignore APT::Get::AutomaticRemove options from config
files such that it unsets the option.

This then means we can do FindB("APT::Get::AutomaticRemove", true) as the
default for APT::Get::AutomaticRemove::Kernels and get the behavior we
want: If you set --no-auto-remove, it is respected as that FindB returns
false; if you don't set it, it will be true.