doxygen2man: Add option to read copyright line from the header file (#415)

• doxygen2man: Add option to read copyright line from the header file

This should help make builds reproducible.

I tried various methods of getting the date, using 'git' is no use as
it could be run from a tarball, using the file date doesn't work either
so this seems a reasonable compromise.

• man: Use SOURCE_EPOCH to make manpage dates reproducible

Also add build-aux/update-copyright.sh to keep header file
copyright lines up-to-date.

All code taken from knet