[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/pngwtran.c b/pngwtran.c
index 762fec3..63d192a 100644
--- a/pngwtran.c
+++ b/pngwtran.c
@@ -236,7 +236,8 @@
  * data to 0 to 15.
  */
 void /* PRIVATE */
-png_do_shift(png_row_infop row_info, png_bytep row, png_color_8p bit_depth)
+png_do_shift(png_row_infop row_info, png_bytep row,
+    png_const_color_8p bit_depth)
 {
    png_debug(1, "in png_do_shift");