for realloc(0) being a valid way to free memory.

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.

alignment and size are both powers of two, and there's no way to
specify an alignment through the malloc() interface anyway.

aligned blocks.

Moved header to include/malloc.h, since we now also provide the
POSIX-like malloc()/free() pair.

Not yet tested.