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.

CHANGES

@@ -4052,7 +4052,7 @@ Version 1.6.0beta16 [March 6, 2012]
     (in fact this is harmless, but the PNG data produced may be sub-optimal).
 
 Version 1.6.0beta17 [March 10, 2012]
-  Fixed PNG_LIBPNG_BUILD_BASE_TYPE definition. 
+  Fixed PNG_LIBPNG_BUILD_BASE_TYPE definition.
   Reject all iCCP chunks after the first, even if the first one is invalid.
   Deflate/inflate was reworked to move common zlib calls into single
     functions [rw]util.c.  A new shared keyword check routine was also added
@@ -4962,7 +4962,7 @@ Version 1.6.13beta01 [July 4, 2014]
   Changed "if defined(__ARM_NEON__)" to
     "if (defined(__ARM_NEON__) || defined(__ARM_NEON))" (James Wu).
   Fixed clang no-warning builds: png_digit was defined but never used.
-    
+
 Version 1.6.13beta02 [July 21, 2014]
   Fixed an incorrect separator ("/" should be "\") in scripts/makefile.vcwin32
     (bug report from Wolfgang S. Kechel).  Bug was introduced in libpng-1.6.11.
@@ -5453,7 +5453,7 @@ Version 1.6.21beta01 [December 11, 2015]
 Version 1.6.21beta02 [December 14, 2015]
   Moved png_check_keyword() from pngwutil.c to pngset.c
   Removed LE/BE dependencies in pngvalid, to 'fix' the current problem
-    in the BigEndian tests by not testing it, making the BE code the same 
+    in the BigEndian tests by not testing it, making the BE code the same
     as the LE version.
   Fixes to pngvalid for various reduced build configurations (eliminate unused
     statics) and a fix for the case in rgb_to_gray when the digitize option
@@ -5517,7 +5517,7 @@ Version 1.6.22beta03 [March 9, 2016]
   Added a common-law trademark notice and export control information
     to the LICENSE file, png.h, and the man page.
   Restored "& 0xff" in png_save_uint_16() and png_save_uint_32() that
-    were accidentally removed from libpng-1.6.17. 
+    were accidentally removed from libpng-1.6.17.
   Changed PNG_INFO_cHNK and PNG_FREE_cHNK from 0xnnnn to 0xnnnnU in png.h
     (Robert C. Seacord).
   Removed dubious "#if INT_MAX" test from png.h that was added to
@@ -5950,7 +5950,7 @@ Version 1.6.32beta09 [August 3, 2017]
   Require cmake-2.8.8 in CMakeLists.txt. Revised symlink creation,
     no longer using deprecated cmake LOCATION feature (Clifford Yapp).
   Fixed five-byte error in the calculation of IDAT maximum possible size.
-  
+
 Version 1.6.32beta10 [August 5, 2017]
   Moved chunk-length check into a png_check_chunk_length() private
     function (Suggested by Max Stepin).