core: Fix handling of duplicate transfer submission/cancellation

core: Fix handling of duplicate transfer submission/cancellation

In the docs, we claim to report LIBUSB_ERROR_BUSY if submitting a
transfer that has already been submitted. Linux was the only backend
that actually checked and reported this, but unfortunately the code
in libusb_submit_transfer() resulted in a segfault. This is because
submission failure would delete the (active) transfer from the
flying_transfers list, and when the transfer finally completes it
would be deleted from the flying_transfers list a second time.

Instead of modifying each backend to check for a busy transfer, this
patch adds a flag to indicate when a transfer is in-flight. The core
library will check this flag and return LIBUSB_ERROR_BUSY immediately.

This patch also modifies libusb_cancel_transfer() to check that a
transfer is in-flight before cancelling and to check that a transfer
has not already been cancelled, returning LIBUSB_ERROR_NOT_FOUND in
both cases.

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>