slightly re-arrange source-code

so all includes and all compat-hacks are grouped together
include v4l2-ctrls if possible (and set HAVE__...)
use goto for cleanup-on-error
make CIDs be based on V4L2LOOPBACK_CID_BASE
setup v4l2_ctrl handler
configure v4l2_ctrls

with a dummy implementation of _s_ctrl() that always returns FAIL
unify set_ctrl
use ctrl_config information in (deprecated) queryctrl
disable deprecated vidioc_*ctrl callbacks
register custom-controls
compat for older kernels (untested)
mark vidioc_*ctrl as unused
set VIDEO_M2M capabilities

github Fixes umlaeute/v4l2loopback#67