Update debian/copyright (Closes: #873740)

I used the following method to extract copyrights and group them, as none of the
tools in https://wiki.debian.org/CopyrightReviewTools produced a useful result:

git grep -I -i -P 'Copyright.*[0-9]+' | grep -v 'The Go Authors' | \
  cut -d: -f1 > /tmp/matching

mkdir /tmp/headers

IFS=$'\n'; for f in $(cat /tmp/matching); do \
  HASH=$(grep -h -P 'Copyright.*[0-9]+' "$f" | md5sum | cut -f 1); \
  grep -l -P 'Copyright.*[0-9]+' "$f" > /tmp/headers/${HASH}; done