Better support for isochronous transfers

[xairy]

Even though Raw Gadget does allow enabling isochronous endpoints, the support is limited: the transfer rate is likely not good enough for practical applications and some isochronous transfer features are not implemented.
Implementing proper support for isochronous transfers requires deep investigation of what kind of Raw Gadget changes are required.

A non-exhaustive list of potentially required changes:

• Support non-blocking I/O (tracked in Support O_NONBLOCK I/O #59);
• Expose frame_number for completed requests.

This issue is also mentioned in the kernel documentation.