cap_app: cs_disp uses capture to send image to display_ipc.
boda_base: declare rt_err_errno() in header for wider use
disp_util: remove unused/disabled pipe_stuffer code/experiment.

we should have enough other asio code examples laying around now.
cap_util: adjust API of on_readable() / read_frame()

they now take a want_frame bool and return a got_frame bool. note:
breaks compilation, usage updates in following commits.
cap_app: cs_disp now does full SlowWrongSort 3-process (cap/proc/disp) demo
conv_pyra: update usages of cap_util.