Some code by Juergen Perlinger refers to the NTP copyright file.
I changed this to an SPDX declaration of the NTP license.

Unity code files contained this: "[Released under MIT License. Please
refer to license.txt for details]". I changed this to an SPDX declaration
of the MIT license.

I did *not* attempt to reduce or tagify other nonstandard license
inclusions, or licenses in files marked "generated", or any license in
a file with more than one license. These will take a separate pass
with some judgment calls.

I also removed some RCS debris.