After all these years, finally we have something reasonable as
a debugserver.  The point of a debugserver is to load (in advance)
a debuginfo file into shared memory, so that dwdebug consumers (like
the target library) can just mmap the data structures pre-allocated
in shared mem.

This is surprisingly hard.  I've looked many times for malloc
replacements that would let me restrict the heap to a fixed base
address, which I have to choose at a "good" location so that it
has high probability of being able to be mmap'd into client processes.
But I never found any that naturally supported it in the API.

Well, silly me, I just hadn't ever read dlmalloc.  That gives me
everything I need.  So my strategy is the usual POSIX shm thing:
shm_open + ftruncate + mmap; then I have ftruncate + mremap to
allocate more core mem to the heap.  Then I create a DL mspace
at a fixed base addr of my choosing, and basically switch heaps
to that one.  Then any future allocations are all done there.

That means, of course, that all the pointers are right in the
structs in the shm, which was the goal.

There's still some work to be done before clients can use this,
but this is the core.