internal functions called by png_do_{read|write}_transformations static. On an
x86-64 DLL build (Gentoo Linux) this reduces the size of the text
segment of the DLL by 1208 bytes, about 0.6%. It also simplifies
maintenance by removing the declarations from pngpriv.h and allowing
easier changes to the internal interfaces.

function definitions before the place where they are called so that
they can be masde static. Move the intrapixel functions and the
grayscale palette builder out of the png?tran.c files. The latter
isn't a transform function and is no longer used internally, and the
former MNG specific functions are better placed in pngread/pngwrite.c

Changed tabs to 3 spaces in png_debug macros and changed '"%s"m'
to '"%s" m' to improve portability among compilers.

to '"%s" m' to improve portability among compilers.
Changed png_free_default() to free() in pngtest.c

reported by clang scan-build.

This reverts to the previous 'static' implementation and works round
the 'unused static function' warning by using PNG_UNUSED().

configurations don't build because of missing contrib/libtests test programs
and overly complex dependencies in scripts/pnglibconf.dfa. This change adds
contrib/conftest/*.dfa files that can be used in automatic build
scripts to ensure that these configurations continue to build.

in pngminim.

to PNG_SET_UNKNOWN_SUPPORTED in pngpread.c; it led to the possibility of
calling png_handle_unknown() twice.

the #ifdef PNG_SET_UNKNOWN_CHUNKS_SUPPORTED/#endif block.
Removed recently-added #ifdef from pngpread.c; moving the prototype
in pngpriv.h fixed the compiler warning that the #ifdef was trying
to solve.

ifdef PNG_SET_UNKNOWN_CHUNKS_SUPPORTE in pngpread.c to agree with
what is in pngpriv.h

(#ifdef PNG_HANDLE_AS_UNKNOWN_SUPPORTED/#endif in pngpread.c)

ctype char checking macros take an unsigned char argument, not a signed char.