Copter: support using WPNav for guided mode position control

[rmackay9]

This resolves, "Dijkstra's in Guided mode not working (reported by Yamaguchi-san)" on the Copter-4.1.0-beta testing issues list by adding a GUID_OPTIONS (=64) to allow using the waypoint navigation controller (aka AC_WPNav, aka SCurves) instead of the position controller to fly to a specified location/position.

The default remains to use the position controller but the difference is:

• waypoint navigation support path planning (BendyRuler, Dijkastra's) while the position controller only supports simple avoidance
• position controller support a much higher update rate (perhaps 50hz vs 3hz)

This implementation includes involves moving the current position controller method to a new "Pos" SubMode and then essentially reverting the "WP" sub mode to use AC_WPNav as it did in Copter-4.0.

Some unrelated changes in this PR:

• GUID_OPTIONS param description is made more human readable
• Removed unused set_desired_velocity_with_accel_and_fence_limits method

This has been tested in SITL including:

1. flew in a horizontal triangular pattern using both controllers to confirm they acted as expected (they did). I also interrupted the vehicle by passing in a new Guided mode target and noticed that the two controllers take different paths (see below).
   

2. Enabled Dijkstra's and confirmed the vehicle flew around the fence
   

[tatsuy]

I've confirmed that Dijkstra's works in SITL.
Thank you for your work!

[lthall]

Really nice :)

[hendjoshsr71]

I played with this as well and it works for terrain following with dijkstra and dijkstras + bend with terrain following.

One note is sometimes it auto yaws to the last dijkstra "constructed wp" instead of where the last command was from. It doesn't always auto yaw though.

[rmackay9]

Shoot, I meant to include this in -rc1