type mismatch causes lwt_unix job hangs

[stijn-devriendt]

To reproduce:
In Lwt_unix.ml, change the following::

  let current_notification_id = ref (0x7FFFFFFF - 1000)

Then run the tests. Test lwt_io_non_block:"many read file" will hang.

Root cause is a type mismatch in the C-code. notification_id is used as int (Int_val) in the job code and long (Long_val) in other cases. The result is that when the notification ID reaches 2^31, notifications from unix jobs are lost. This is because the 0x80000000L is cast to int, yielding INT_MIN. Later on, this value is cast back to long with sign extension (0xFFFFFFFF80000000) to generate the notification. This value is not found in the notifications hashtable, causing loss of notifications and thus a hang of the affected Lwt tasks.

All uses of notification id's should be long to match the ocaml value's size.