Support for Multi-Arch

Add support for the Multi-Arch control header (https://wiki.ubuntu.com/MultiarchSpec )

Contains one gross monkey patch workaround as JDeb does not support arbitrary headers and didn't appear to have a good place for me to sneak in without other measures/re-implementation, etc. If/when my small change gets accepted, we can pull that hack out and move to that JDeb version to cleanse :)