[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.
diff --git a/pngwio.c b/pngwio.c
index cc69b84..31309b3 100644
--- a/pngwio.c
+++ b/pngwio.c
@@ -30,10 +30,11 @@
  */
 
 void /* PRIVATE */
-png_write_data(png_structp png_ptr, png_bytep data, png_size_t length)
+png_write_data(png_structp png_ptr, png_const_bytep data, png_size_t length)
 {
+   /* NOTE: write_data_fn must not change the buffer! */
    if (png_ptr->write_data_fn != NULL )
-      (*(png_ptr->write_data_fn))(png_ptr, data, length);
+      (*(png_ptr->write_data_fn))(png_ptr, (png_bytep)data, length);
 
    else
       png_error(png_ptr, "Call to NULL write function");