Replace license boilerplate with single line SPDX comments.

This gets rid of existing copyright templates in favor of just the
one-liner SPDX (Software Package Data Exchange) notice. All files are
licensed under the GPL-2.0, so the same tag is added to each file. No
copyright is changed by this commit.

Several files (mostly the very short ones) did not contain a license
text so far. By default all files without license information are
under the default license of this package, which is GPL version 2.
This commit adds the missing SPDX line so that now all files except
dss.css, index.html.in, INSTALL, NEWS and README have it.

We also remove author and copyright year, since the author is the same
everywhere, and the year hasn't been updated any more since at least
six years. Accurate information is available from the git log.

The COPYING file can also be removed because the license text at
https://spdx.org/licenses/GPL-2.0.html is immutable.