defined in vsprintf.h.  (This may change, since vsprintf.h is a
non-standard name, but for now it's the one to use.)

There should be no need to include vsprintf.h just for DBG() statements,
since include/compiler.h forces it in for a debug build anyway.

for use by malloc().

This breaks the image-loading code (which previously used the heap to
allocate the buffer for downloading the image), but that's not a major
concern since I'm going to tear out all the image formats within the next
couple of days anyway.  Byebye, NBI!  :)