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.

fakelibusb_test.go

@@ -149,11 +149,11 @@ func (f *fakeLibusb) setAlt(d *libusbDevHandle, intf, alt uint8) error {
 	return nil
 }
 
-func (f *fakeLibusb) alloc(_ *libusbDevHandle, _ *EndpointDesc, _ time.Duration, _ int, buf []byte, done chan struct{}) (*libusbTransfer, error) {
+func (f *fakeLibusb) alloc(_ *libusbDevHandle, _ *EndpointDesc, _ time.Duration, _ int, bufLen int, done chan struct{}) (*libusbTransfer, error) {
 	f.mu.Lock()
 	defer f.mu.Unlock()
 	t := newFakeTransferPointer()
-	f.ts[t] = &fakeTransfer{buf: buf, done: done}
+	f.ts[t] = &fakeTransfer{buf: make([]byte, bufLen), done: done}
 	return t, nil
 }
 func (f *fakeLibusb) cancel(t *libusbTransfer) error { return errors.New("not implemented") }
@@ -164,6 +164,7 @@ func (f *fakeLibusb) submit(t *libusbTransfer) error {
 	f.submitted <- ft
 	return nil
 }
+func (f *fakeLibusb) buffer(t *libusbTransfer) []byte { return f.ts[t].buf }
 func (f *fakeLibusb) data(t *libusbTransfer) (int, TransferStatus) {
 	f.mu.Lock()
 	defer f.mu.Unlock()