Fix WaitHandle usage in delegates.

Fix WaitHandle usage in delegates.
* object-internals.h (mono_wait_handle_get_HANDLE): Declare.
* object.c (mono_wait_handle_new): Use the property set method to
initialize the handle.
(mono_wait_handle_get_handle): New.
* threadpool.c (mono_async_invoke): Use it.
* threads.c (ves_icall_System_Threading_WaitHandle_WaitAll_internal): Likewise.
(ves_icall_System_Threading_WaitHandle_WaitAny_internal): Likewise.

svn path=/trunk/mono/; revision=69876