[devel]Made all API functions that have const arguments and constant string

literal pointers declare them (John Bowler).
Many APIs did not change their arguments but were not declared using
PNG_CONST.  This change corrects this.  In a few cases APIs that return
constant string literal pointers have also been changed to declare this.
Unlike the argument change this may require app changes; however the
results could never be written to (the app would crash on some platforms
where strings are not writable), so this seems advisable.

ANNOUNCE

@@ -1,5 +1,5 @@
 
-Libpng 1.5.0beta42 - August 13, 2010
+Libpng 1.5.0beta42 - August 11, 2010
 
 This is not intended to be a public release.  It will be replaced
 within a few weeks by a public version or by another test version.
@@ -226,7 +226,7 @@ version 1.5.0beta24 [May 7, 2010]
     offset of the png_ptr->rowbuf pointer into png_ptr->big_row_buf.
   Added more blank lines for readability.
 
-version 1.5.0beta25 [August 13, 2010]
+version 1.5.0beta25 [August 11, 2010]
   In pngpread.c: png_push_have_row() add check for new_row > height
   Removed the now-redundant check for out-of-bounds new_row from example.c
 
@@ -344,8 +344,10 @@ version 1.5.0beta41 [August 11, 2010]
   Don't try to use version-script with cygwin/mingw.
   Revised contrib/gregbook to work under cygwin/mingw.
 
-version 1.5.0beta42 [August 13, 2010]
+version 1.5.0beta42 [August 11, 2010]
   Add .dll.a to the list of extensions to be symlinked by Makefile.am (Yaakov)
+  Made all API functions that have const arguments and constant string
+    literal pointers declare them (John Bowler).
 
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