prototypes for most png_get_*() functions.

in pngvalid.c

objective of simplifying and improving the cosmetic appearance of png.h.

with the objective of improving the cosmetic appearance of png.h.

  Added some commentary about PNG_EXPORT in png.h and pngconf.h

which returned the wrong type (png_uint_32 instead of png_uint_16).

Moved reading of file signature into png_read_sig (Cosmin)
Fixed atomicity of chunk header serialization (Cosmin)
Added test for io_state in pngtest.c (Cosmin)
Added "#!/bin/sh" at the top of contrib/pngminim/*/gather.sh scripts.