- Make two variable name modification to clarify units: altitude -> altitude_ft

- Make two variable name modification to clarify units: altitude -> altitude_ft
  and tgt_altitude -> tgt_altitude_ft.  Also fix a comment in AIBase.hxx
  indicating that the altitude is in meters, even though the usage throughout the
  code was most definitely feet.

- In AIMultiplayer.cxx, update the altitude_ft variable so that the altitude
  is reported correctly in the entity's property subtree.

- In AIMultiplayer.cxx, compute a velocity value in kts to fill in the speed
  entry in the entity's property subtree.  Note, this is not an earth centered
  reference speed, not an indicated speed and not a speed relative to the local
  airmass (that would be much harder to do.)