bundle2.unbundlepart: decouple mandatory from parttype Encoding whether or not a part is mandatory in the capitalization of the parttype is unintuitive and error-prone. This sequence of patches separates these concerns in the API to reduce programmer error and pave the way for a potential change in how this information is transmitted over the wire. This patch separates the two pieces of information when reading the part header so that it's unnecessary to know how they were combined during transmission.