* gendef (_sigfe): Save arguments before calling yield so they don't
	get clobbered.  Explain why.
	* Revert to old malloc.
	* mmap.cc (MMAP_STORAGE_LOW): Set higher.  Add comment.
	(MMAP_STORAGE_HIGH): Add comment.
	(class mmap_allocator): Change comment.  Add mmap_current_low member.
	(mmap_allocator::mmap_allocator): Define default constructor.
	(mmap_allocator::alloc): Change to a round-robin mechanism.  Add or
	change comments accordingly.