Clean up packaging (from Nicolas Boulenguez)

Ben Hutchingsrequested to merge
benh/firmware-nonfree:bug1064620 into debian/latest
  • d/rules: include /usr/share/dpkg/pkg-info.mk
  • gencontrol.py: remove unused process_template methods
  • gencontrol.py: open files with context managers
  • d/rules: remove gencontrol.py from GENCONTROL

Closes: #1064620

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