Add flag to funptr to register calling threads with C.

[yallop]

C threads that call into OCaml must first be registered with the runtime using caml_c_thread_register. This is sometimes a difficult requirement to meet when when calling into OCaml through a funptr, since threads may be created inside third-party code that it is not possible to modify.

It'd be convenient to have an option to funptr to wrap calls in caml_c_thread_register and caml_c_thread_unregister, much as the runtime_lock argument makes it possible to acquire and release the runtime lock when calling back into OCaml from C.