Corrected copyright notices and info about adding copyright info

Until now, the files where the people were meant to change didn't have a
proper copyright notice (for example `Copyright (C) YOUR NAME.'). This was
wrong because the license does not convey copyright ownership. So the name
of the file's original author must always be included and when people
modify it (and add their own copyright-able modifications).

With this commit, the file's original author (and email) are added to the
copyright notice and when more than one person modified a file, both names
have their individual copyright notice.

Based on this, the description for adding a copyright notice in
`README-hacking.md' has also been modified.