Allocate libusb buffers in C (#11)

* add alloc/free_transfer_and_buffer. Manages the buffer memory on the C
side.

* switch libusb.go to use the new alloc/free_transfer_and_buffer. Add
a new buffer() call to get access to the allocated buffer as a Go slice.

* Fake USB transfer uses the new alloc/free/buffer interface.

* Switch to the new libusb.alloc signature, where libusb owns the buffer.

* newUSBTransfer now allocates a separate buffer, do a copy on
endpoint.transfer.

* newUSBTransfer will now allocate it's own buffer.

* Enable autodetach in rawread.

rawread/main.go

@@ -140,6 +140,9 @@ func main() {
 	}
 	dev := devs[0]
 
+	log.Print("Enabling autodetach")
+	dev.SetAutoDetach(true)
+
 	log.Printf("Setting configuration %d...", *config)
 	cfg, err := dev.Config(*config)
 	if err != nil {