Registration of C threads from callbacks

[yallop]

This pull request adds a new argument, ~thread_registration, to funptr. Passing ~thread_registration:true indicates that the C thread that calls into OCaml through the function pointer should be registered with the OCaml runtime (via caml_c_thread_register).

C threads so registered are automatically unregistered (via caml_c_thread_unregister) when they exit.

This functionality is currently unavailable on Windows, which apparently provides little support for running functions on thread termination. Perhaps it's possible to implement using RegisterWaitForSingleObject; patches are, of course, very welcome! (@fdopen: if you have time to look at this, your help would be greatly appreciated.)

Here's a quick example of the thread_registration in action, via a binding to pthread_create:

(* thread_example.ml *)
open Ctypes

let pthread_t = ulong

let pthread_create = Foreign.foreign "pthread_create"
    (ptr pthread_t @-> ptr void @->
     Foreign.funptr ~thread_registration:true ~runtime_lock:true
         (ptr void @-> returning (ptr void)) @->
     ptr void @-> returning int)
    ~release_runtime_lock:true

let pthread_join = Foreign.foreign "pthread_join"
    (pthread_t @-> ptr (ptr void) @-> returning int)
    ~release_runtime_lock:true

let callback p =
  print_endline "Hello, threaded world.";
  flush stdout;
  null

let _ = begin
  let thread_id = allocate_n pthread_t 1 in
  let _ = pthread_create thread_id null callback null in
  let _ = pthread_join !@thread_id (coerce (ptr void) (ptr (ptr void)) null) in
   ()
end

$ ocamlfind opt -package ctypes.foreign -thread -linkpkg  thread_example.ml
$ ./a.out 
Hello, threaded world.

This closes #269.