flight.control.altitude: saner use of control law
userinput.ppm.decode fixup

* kinda important!
deprecate old flight.userinput
userinput: add multiplexing for rcoverride input

* still not working correctly, for some reason the mux
  never gets an rcoverride message from the GCS even when
  mavproxy is sending them. need to investigate further
flight.userinput: bugfixes to get mavlink input to work