Windows support for registration of C threads from callbacks

[fdopen]

Follow up to #420, fix for #421.
Explanation is inside the source as comment.

[yallop]

Wonderful! Thank you.

(We'll need tests for this eventually. Unfortunately there's not even a test for the pthreads version yet. But it's reported to work.)

[fdopen]

For the windows case I had of course written a basic test that I could add to the test suite (a dll that spawns a few threads in parallel, each calling back several times to OCaml with its thread id)

The problem is that it's only easy to test that there is no segfault (as without thread registration). Testing that caml_c_thread_unregister/caml_thread_remove_info are called is however non-trivial. The compiler specific instruction https://github.com/ocamllabs/ocaml-ctypes/blob/50b39eed6fbf13f269c1460f9725dc86ea1b7063/src/ctypes-foreign-threaded/foreign_threaded_stubs.c#L54 could fail silently in the future (e.g. if support for a different compiler is added). But external tools (e.g. a gdb-script) are difficult to maintain.

[yallop]

The problem is that it's only easy to test that there is no segfault

Right. But I think that's still useful, even if it doesn't tell you everything you'd like to know.