outside of an unknown-chunk block in png.h because they are also
needed for other uses.

 (libpng-manual.txt still to be updated, see png.h for documentation.)

 This change adds internal APIs to allow png_warning messages to have parameters
 without requiring the host OS to implelment snprintf.  As a side effect the
 dependency of the RFC1132 code on stdio is removed and PNG_NO_WARNINGS does
 actually work now.