and revised libpng style to mention not using the TAB character.

and always use png_calloc().  The macros were only there for testing,
since libpng-1.4.0beta48 and no problems have been reported.

Always use local arrays, which has been the default behavior
for a long time.

except in pngconf.h where they can be used to request that
PNG_FEATURE_SUPPORTED not be defined.

It does not seem to promise any benefit to libpng.

To try this, configure libpng using PNG_USE_MEMSET_LOOP.  Initial testing
with pngcrush was not promising, although it does save a hundred bytes or
so of executable size.