core: fix lowmem malloc

core: fix lowmem malloc

a) initialize *all* the head nodes
b) the lowmem heap needs to be paragraph (16 byte) aligned
c) use a dedicated tag value for headnodes.  The type code should be
   removed and replaced with special tag values for head node and
   free.
d) for libcom32 code called from the core, do NOT present a
   usable bounce buffer; we should only ever use lmalloc/lfree.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>