Improve path safety

[leijurv]

Right now, every Movement just expects that the player is standing in src. However when things get screwed up that isn't the case. And there are some really crappy heuristics that it uses. For example traverse jumps when its Y level is below what it expects it to be even though that's NOT its job. Instead of canceling the path if it's been more than 2 blocks away for more than 5 seconds, it should immediately cancel it when the player is off the path. This also helps if the real player is trying to take over (like if there's a mob) and doesn't want to type cancel, they can divert away from the path.