Set Copyright years and make them easier to update

I replaced git-set-file-times with an improved version that I wrote
recently (in python3). A new script uses it to figure out the
last-modified year for each *.[ch] file and updates its copyright.
It also puts the latest year into the latest-year.h file for the
output of --version.
diff --git a/rounding.c b/rounding.c
index c4c29b3..bf2ecd3 100644
--- a/rounding.c
+++ b/rounding.c
@@ -1,7 +1,7 @@
 /*
  * A pre-compilation helper program to aid in the creation of rounding.h.
  *
- * Copyright (C) 2007-2019 Wayne Davison
+ * Copyright (C) 2007-2020 Wayne Davison
  *
  * This program is free software; you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by