deb: use product name as upstream
deb: don't repeat "Copyright" in Copyright field

Reported by Hideki Yamane. Thanks!!!
deb: add description about the complete text of GPL on Debian

Reported by Hideki Yamane. Thanks!!!
deb: add people who contribute to deb packaging

Reported by Hideki Yamane. Thanks!!!
deb: indent Copyright field for readability

Reported by Hideki Yamane. Thanks!!!