in memory I2C mux state does not always reflect true state

[bcantrill]

Fixes #1413; draft comment:

Addresses #1413 by:

- Eliminating the possibility of having in-memory state not reflect
  hardware state by intoducing an explicit state (MuxState::Unknown)
  denoting unknown mux state for a bus (and therefore necessitating
  mux initialization on any subsequent transaction)

- Making sure that any failed attempt to set mux state (e.g., in
  initial configuration) indicates that the bus is in
  MuxState::Unknown

- Fixing a source of initial configuration failures due to reset
  during an I2C transaction by correcting the SCL wiggling code to
  correctly clock through the zombie transaction and assert a STOP
  condition

- Greatly expanding the ring buffer holding I2C errors (which pushes
  us over a memory threshold -- but this is clearly a price worth
  paying)

- Making the ring buffer much more explicit as to observed errors

- Having reset of the controller reset all muxes on a bus rather
  than merely the mux that was involved in the transaction that
  induced the need for a reset -- and have all such resets set the
  state to be MuxState::Unknown

Additionally, this eliminates the selected_mux_segment entry point,
which has outlived its usefulness.