PNG_UNKNOWN_CHUNKS_SUPPORTED should be PNG_HANDLE_AS_UNKNOWN_SUPPORTED

that was recently added to png_write_find_filter() in pngwutil.c

into png.h and revised the documenatation (in the libpng style section) about
placement of prototypes.

in png_set_text_2() (bug introduced in libpng-0.95, discovered by Evan Rouault)

(suggested by Peter Breitenlohner)

from autoconf-2.62

to be consistent with the rest of the source files.

in pngset.c

outside of the PNG_READ|WRITE_ANCILLARY_CHUNK_SUPPORTED macros and
updated CHANGES and ANNOUNCE.  The definitions were actually relocated
in the previous commit but I forgot to mention it.