v4l2 backend: Y16 default behavior fixed & FOURCC setting added
v4l2 backend: Y16 default behavior fixed & FOURCC setting added

1) Cameras started with Y16 (V4L2_PIX_FMT_Y16) format via v4l2 backend will now exhibit default camera behavior, i.e. convert the 16-bit image to BGR as with all other formats. 16-bit 1-channel output will now only be produced for Y16 if CV_CAP_PROP_CONVERT_RGB is set to "false" using VideoCap::set method.
2) v4l2 videoio backend now supports setting CV_CAP_PROP_FOURCC explicitly (icvSetPropertyCAM_V4L function in cap_v4l.cpp), allowing users to manually set the codec on cameras that support multiple codecs.

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