[PATCH] Add API for releasing zero-copy USB buffers

Hi, I was trying to take advantage of the async zero-copy buffer feature in librtlsdr, but I found that the automatic resubmission of them made it hard to make use of them safely. This patch adds a couple function to manage buffers in a more advanced manner, so that latency and reuse can be tracked precisely. I also added some documentation. I can occasionally make a lot of mistakes, so if anybody has time to take a brief glance through, it would be really great. Karl