is now gPXE-specific.  (The standard malloc() et al have been in stdlib.h
for a while).

Add free memory counter.

function prototypes (e.g. malloc_dma()) in malloc.h.

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

aligned blocks.

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

Not yet tested.