cam: Minor opt_cam.h cleanup

sys/cam/cam.h includes opt_cam.h, so none of the clients need to do
this. cam.h does all the right dancing to conditionally include
opt_cam.h only when it makes sense. It generally only matters when
cam_debug.h is included (it must be included before that). Many of the
stray opt_cam.h includes were after cam_debug.h which would be a problem
were it not included in cam/cam.h. The other users of CAM options that
aren't debug all already include cam/cam.h.

Also trim unneeded sys/cdefs.h files from the files touched.

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