Linux: Add support for the new URB_BULK_CONTINUATION flag

Linux: Add support for the new URB_BULK_CONTINUATION flag

Add support for the new USBDEVFS_URB_BULK_CONTINUATION flag to libusb.

This flag, which is expected to be available in usbfs starting with
kernel 2.6.32, allows the kernel to cancel multiple URBs upon receipt
of a short packet.  This capability allows libusb to preserve data
integrity of large bulk transfers that are split into multiple URBs.
Without this support, these URBs must be canceled in userspace upon
receipt of a short packet, a race condition against future transfers
which might partially fill these canceled URBs.

This patch automatically detects whether a supported kernel is present
and enables the use of the flag when possible.

[dsd: tweaks to supported kernel detection, and some inline
 documentation of this mechanism]