(psep_open) allow hidraw forwarding

[letoram]

Core arcan (for led), arcan_vr and afsrv_decode all need to open USB devices, and doing the normal usb juggling is not very pleasant.

Plan:

• [ ] modify hidraw + libusb to open with fd
• [ ] add vid/pid requests to psep_open
• [ ] forward as DEVICEHINT to client
• [ ] add client event for a DEVICEREQ
• [ ] add Lua logic for ack:ing that
• [ ] update psep_open to stitch all this together

Notes: The libusb stage will be the most painful. Afaict there is no easy hack to just get the fd into the device structure. ChromeOS, Android and others all have patches that achieve this, but they are not upstreamed for some reason.