[libpng16] Updated manual to mention that warnings about iTXt language tag

and translated keyword are no longer issued when these are empty, which
is allowed by the PNG specification.
[libpng16] Added "/* SAFE */" comments in pngrutil.c and pngrtran.c

where warnings may be erroneously issued by code-checking applications.
Revised manual about changes in iTXt chunk handling made in libpng-1.6.0.
[libpng16] Revised "/*SAFE*/" comment in pngrutil.c
[libpng16] Imported from libpng-1.6.3.tar