darwin: fix race condition on device capture

[osy]

If two threads try to capture at the same time, the result can be inconsistent. For example, a process can have two different contexts to libusb and both call libusb_reset_device.

[Mr-Bossman]

I wonder if this fixes https://github.com/libusb/libusb/issues/1635 I'll check in a bit

[seanm]

@osy BTW you may be interested in https://github.com/libusb/libusb/pull/1419

In fact I think I'll revive that effort, we could probably get some of it in first...

[mcuee]

@osy

Would you please address the comments from @seanm? Thanks.