Redo the locking, so that it may actually work. Now locking is done at

Redo the locking, so that it may actually work.  Now locking is done at
the malloc/free level, not within the heap abstraction, and there's a
separate lock to control sbrk access.

Also, get rid of the separate `unmap_free_area' function in free.c, and
just put the code in the `free' function directly, which saves a bunch
of space (even compared to using an inline function) for some reason.