usbip: extract URBEngine + userspaceURBSession from darwin backend

Both the existing darwin backend and the upcoming windows backend
drive USB devices from user space (IOUSBHost CGO calls vs. VBoxUSB
IOCTLs). Refactor the per-attachment URB loop out of host_darwin.go
into a platform-agnostic userspaceURBSession that talks to a URBEngine
interface; the darwin-specific dispatch becomes a 30-line
darwinIOUSBHostEngine. Linux's kernelHandoffSession is untouched.

Move hex8 into shared.go and add usbipStatusEIO so the shared session
does not depend on golang.org/x/sys/unix (Windows has no equivalent).

service/usbip/data_protocol.go

@@ -26,6 +26,7 @@ const (
 	maxUSBIPIsoPackets       = 1024
 	nonIsoPacketCount        = -1
 	usbipTransferFlagIsoASAP = 0x0002
+	usbipStatusEIO           = -5
 	usbipStatusECONNRESET    = -104
 )