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/checksum-icc.c

@@ -2,8 +2,6 @@
  *
  * Copyright (c) 2013 John Cunningham Bowler
  *
- * Last changed in libpng 1.6.0 [February 14, 2013]
- *
  * This code is released under the libpng license.
  * For conditions of distribution and use, see the disclaimer
  * and license in png.h
@@ -11,8 +9,8 @@
  * Generate crc32 and adler32 checksums of the given input files, used to
  * generate check-codes for use when matching ICC profiles within libpng.
  */
-#include <stdio.h>
 
+#include <stdio.h>
 #include <zlib.h>
 
 static int

contrib/tools/cvtcolor.c

@@ -1,7 +1,4 @@
-/*-
- * convert.c
- *
- * Last changed in libpng 1.6.0 [February 14, 2013]
+/* convert.c
  *
  * COPYRIGHT: Written by John Cunningham Bowler, 2013.
  * To the extent possible under law, the author has waived all copyright and
@@ -10,6 +7,7 @@
  *
  * Convert 8-bit sRGB or 16-bit linear values to another format.
  */
+
 #define _ISOC99_SOURCE 1
 
 #include <stdlib.h>

contrib/tools/genpng.c

@@ -1,4 +1,4 @@
-/*- genpng
+/* genpng
  *
  * COPYRIGHT: Written by John Cunningham Bowler, 2015.
  * Revised by Glenn Randers-Pehrson, 2017, to add buffer-size check.
@@ -69,6 +69,7 @@
  * joins are mitres; the outside of the lines are continued to the point of
  * intersection.
  */
+
 #include <stddef.h>
 #include <stdlib.h>
 #include <string.h>

contrib/tools/makesRGB.c

@@ -1,6 +1,4 @@
 /* makesRGB.c -- build sRGB-to-linear and linear-to-sRGB conversion tables
- *
- * Last changed in libpng 1.6.0 [February 14, 2013]
  *
  * COPYRIGHT: Written by John Cunningham Bowler, 2013.
  * To the extent possible under law, the author has waived all copyright and
@@ -14,6 +12,7 @@
  * approximation to the 8-bit sRGB encoded value.  Calculate the error in these
  * tables and display it.
  */
+
 #define _C99_SOURCE 1
 #include <stdio.h>
 #include <math.h>

contrib/tools/png-fix-itxt.c

@@ -1,8 +1,6 @@
-
-/* png-fix-itxt version 1.0.0
+/* png-fix-itxt
  *
  * Copyright 2015 Glenn Randers-Pehrson
- * Last changed in libpng 1.6.18 [July 23, 2015]
  *
  * This code is released under the libpng license.
  * For conditions of distribution and use, see the disclaimer

contrib/tools/pngcp.c

@@ -2,8 +2,6 @@
  *
  * Copyright (c) 2016 John Cunningham Bowler
  *
- * Last changed in libpng 1.6.24 [August 4, 2016]
- *
  * This code is released under the libpng license.
  * For conditions of distribution and use, see the disclaimer
  * and license in png.h
@@ -15,6 +13,7 @@
  * For a more extensive example that uses the transforms see
  * contrib/libtests/pngimage.c in the libpng distribution.
  */
+
 #include "pnglibconf.h" /* To find how libpng was configured. */
 
 #ifdef PNG_PNGCP_TIMING_SUPPORTED
@@ -778,7 +777,7 @@ static void
 set_opt_string(struct display *dp, unsigned int sp)
    /* Add the appropriate option string to dp->curr. */
 {
-   dp->stack[sp].opt_string_end = set_opt_string_(dp, sp, dp->stack[sp].opt, 
+   dp->stack[sp].opt_string_end = set_opt_string_(dp, sp, dp->stack[sp].opt,
       options[dp->stack[sp].opt].values[dp->stack[sp].entry].name);
 }
 

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>

contrib/tools/sRGB.h

@@ -1,7 +1,4 @@
-/*-
- * sRGB.h
- *
- * Last changed in libpng 1.6.0 [February 14, 2013]
+/* sRGB.h
  *
  * COPYRIGHT: Written by John Cunningham Bowler, 2013.
  * To the extent possible under law, the author has waived all copyright and
@@ -16,6 +13,7 @@
  * (in fact the source of the numbers is the wikipedia article at
  * https://en.wikipedia.org/wiki/SRGB).
  */
+
 static double
 sRGB_from_linear(double l)
 {