bug fixes:
- fixed servo output code (init and isr)
- added all-servo sweep to dump_sensors()
- cleaned up ailr_out for AIL_SINGLE mode. delay till output stage.

will need to revisit
- servo out code again. instead of fixed pulse width, perhaps use the end time to compute next wait interval. but check in this fixed version first.
- rx in code, perhaps have common code (macro) for rx in (now in 3 places) and set ailr_in = ail_in in AIL_SINGLE mode there. then the rest of the code assumes AIL_DUAL mode.