Use MIT license instead of CC0

Use MIT license instead of CC0

CC0 has continued to fall out of favor due to the patents clause
(https://lwn.net/ml/fedora-legal/CAC1cPGw1xScGAXo-0NRs92zFB7ptRxTt=oCYi0BxfZDfAgUtYQ@mail.gmail.com).
Years ago I released some source files in this project (not the whole
project) under CC0.  Use the MIT license for these files instead.

Note that this requires claiming copyright on the latest version of
these files.  Of course, previous versions of these files remain public
domain where legally recognized; this is *not* in any way an attempt to
"revoke" the public domain status of previous versions.