Autopilot improvements

[tdammers]

Improvements:

• Better vertical-speed-to-pitch control: aircraft no longer sinks too fast at low speeds during IFR approach
• Better tracking of FMS/LNAV: instead of just homing in on the waypoint, or flying parallel to the leg bearing, the autopilot now intercepts the FMS track just like it would intercept a NAV radial
• Turn anticipation for LNAV (needs some work still: autopilot now just flips to the next waypoint if the current one is a fly-by waypoint and closer than a standard 1-minute turn radius: this is correct for 90-degree turns, but too early for shallower turns, and too late for sharper turns)
• I had some experimental Nasal code in there for holds, but this depends on a custom patch to FG itself, and didn't actually work, so I haven't bothered implementing any of that in the more XML-based rewrite of the autopilot stuff

[tdammers]

This should probably not get merged in this state.