Clean up comments and whitespace characters in source files

Remove all remaining "last changed" version info from source comments.
(The version control system maintains this information automatically.)

Delete the trailing whitespace characters.

contrib/tools/pngfix.c

@@ -1,6 +1,5 @@
 /* pngfix.c
  *
- * Last changed in libpng 1.6.31 [July 27, 2017]
  * Copyright (c) 2014-2017 John Cunningham Bowler
  *
  * This code is released under the libpng license.
@@ -10,6 +9,7 @@
  * Tool to check and fix the zlib inflate 'too far back' problem.
  * See the usage message for more information.
  */
+
 #include <stdlib.h>
 #include <stdio.h>
 #include <string.h>