daklib/checks.py: Reject binary packages using `unknown` as section

Previously, these would have gone through (and probably ended up in NEW where it would then be manually rejected). With this change, we just reject them early.

This is mostly useful if dpkg were to automatically populuate Section when it was omitted. Related:

https://lists.debian.org/debian-devel/2024/12/msg00328.html

Note that previously, this case would have the binary not have a Section, which rejected a bit further up.