HOOKIN

infinity-thread-db.c will enable thread-debugging per-pspace,
linux-thread-db.c tracks libthread_db instances by PID:

  static struct thread_db_info *get_thread_db_info (int pid)

What I did in the original gbenson/infinity branch was right.

* Have infinity-thread-db.c watch functions arriving and collect them
  in some per-pspace struct (which has some is_complete field which
  the i8func observers maintain).

* Whenever linux-thread-db.c sees a new objfile (hopefully AFTER
  infinity(-notes).c saw it!) it's thread_db_new_objfile observer
  calls check_for_thread_db which calls thread_db_load.

* thread_db_load can grab a (fake, per-pid/per-pspace) "dlopen" handle
  from infinity-thread-db.c that it can pass to thread_db_load_* as
  before.  Also infinity-thread-db.c should have some hacky dlsym
  and dlclose wrappers. (maybe dlopen too, for completeness).

Boom, done.