for consistency, and remove some useless tests (Alexey Petruchik).

tests/pngunknown-sAPI so it's temporarily marked SKIP.

properly from scripts/pnglibconf.dfa and pnglibconf.h.prebuilt.

Visual Studio (Sergey Kosarevsky)

and 1-million-row default limits in pnglibconf.dfa, that can be reset
by the user at build time or run time.  This provides a more robust
defense against DOS and as-yet undiscovered overflows.

Display user limits in the output from pngtest.

Work around more Coverity-scan dead-code warnings.

pngset.c to avoid warnings about dead code.

values that are exactly 2^bit_depth, and work on 16-bit platforms.