[devel] Removed spurious tabs, shorten long lines (no source change)

Also added scripts/chkfmt to validate the format of all the files that can
reasonably be validated (it is suggested to run "make distclean" before
checking, because some machine generated files have long lines.)
Reformatted the CHANGES file to be more consistent throughout.

contrib/gregbook/README

@@ -63,9 +63,9 @@ BUILD INSTRUCTIONS
 
  - Prerequisites (in order of compilation):
 
-      - zlib		http://zlib.net/
-      - libpng		http://www.libpng.org/pub/png/libpng.html
-      - pngbook		http://www.libpng.org/pub/png/book/sources.html
+      - zlib            http://zlib.net/
+      - libpng          http://www.libpng.org/pub/png/libpng.html
+      - pngbook         http://www.libpng.org/pub/png/book/sources.html
 
      The pngbook demo programs are explicitly designed to demonstrate proper
      coding techniques for using the libpng reference library.  As a result,
@@ -165,19 +165,19 @@ RUNNING THE PROGRAMS:  (VERY) BRIEF INTRO
      rpng is a simple PNG viewer that can display transparent PNGs with a
      specified background color; for example,
 
-	rpng -bgcolor \#ff0000 toucan.png
+        rpng -bgcolor \#ff0000 toucan.png
 
      would display the image with a red background.  rpng2 is a progressive
      viewer that simulates a web browser in some respects; it can display
      images against either a background color or a dynamically generated
      background image.  For example:
 
-	rpng2 -bgpat 16 toucan.png
+        rpng2 -bgpat 16 toucan.png
 
      wpng is a purely command-line image converter from binary PBMPLUS/NetPBM
      format (.pgm or .ppm) to PNG; for example,
 
-	wpng -time < toucan-notrans.ppm > toucan-notrans.png
+        wpng -time < toucan-notrans.ppm > toucan-notrans.png
 
      would convert the specified PPM file (using redirection) to PNG, auto-
      matically setting the PNG modification-time chunk.