Fps fix

These 3 patches fix some minor annoying compatibility with the v4l2 subsystem.  This makes uvc_cam work with bttv based capture cards (and probably many more).  There were 2 bugs:  First, not all v4l2 devices support changing (or even querying) the framerate, so that ioctl failing is not an error. Second, some drivers  (bttv specifically) expect that mmaped image buffers are maped read and write.

-James