src/: IMG_HOSTED no longer affects the img API at all.
src/img.c: Don't duplicate the IMG_API_VERSION defaulting logic from
img.h as we get it just by including img.h first.
NEWS,src/img.c: Add missing definition of max() macro when built for
use outside of Survex.
NEWS,src/img.c: img.c can now be compiled as C++ as well as as C.
src/img.c: Don't segfault on img_close() in non-hosted mode.
src/img.c: Use osnew() to simplify code a little.
src/img.c: Improve fputsnl() for non-HOSTED mode to return a value.