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.

endpoint.go

@@ -89,17 +89,23 @@ func (e *endpoint) transfer(buf []byte) (int, error) {
 		return 0, nil
 	}
 
-	t, err := newUSBTransfer(e.h, &e.Desc, buf, e.Timeout)
+	t, err := newUSBTransfer(e.h, &e.Desc, len(buf), e.Timeout)
 	if err != nil {
 		return 0, err
 	}
 	defer t.free()
+	if e.Desc.Direction == EndpointDirectionOut {
+		copy(t.data(), buf)
+	}
 
 	if err := t.submit(); err != nil {
 		return 0, err
 	}
 
 	n, err := t.wait()
+	if e.Desc.Direction == EndpointDirectionIn {
+		copy(buf, t.data())
+	}
 	if err != nil {
 		return n, err
 	}