[io-layer] Fix two race conditions, one on FD handle creation and one on interrupt...

[io-layer] Fix two race conditions, one on FD handle creation and one on interrupt install.

mono_w32handle_new_fd: We must hold scan_mutex while initializing an unallocated handle otherwise
mono_w32handle_foreach might think it's a valid handle.

mono_w32handle_timedwait_signal_handle: We must increment the refcount before installing the interrupt handle
as the other end can be called right after we finish installing and would trigger a failure.