AP_Mount: implement user defined angle limits (especially for Siyi)

[rmackay9]

This resolves #13815, partially resolves issue #21547 and should also fix the Siyi A8 ROI problem reported by @kellyschrock (see the Copter-4.4 issues list).

The changes include:

1. AP_Mount::set_angle_targets() limits the roll, pitch and (sometimes) yaw angles to be sure they are within the user defined limits (e.g. MNT1_ROLL_MIN/MAX, MNT1_PITCH_MIN/MAX, MNT1_YAW_MIN/MAX). Note that yaw is only checked when it is in body-frame. We sadly cannot limit the yaw when it is defined in earth-frame at the moment it is received because the yaw of the vehicle will likely change over time. Instead the individual backends must enforce earth-frame yaw limits.
2. AP_Mount_Siyi enforces the earth-frame yaw limit just before it sends the latest rate control command to the gimbal. Somewhat interestingly we also need to change the lock/follow (aka earth-frame/body-frame) setting sent to the gimbal to body-frame (aka follow) or the gimbal itself will stop controlling its attitude if it is rotated over 180degrees. This seems to me to be a bug within the Siyi gimbals themselves.

This has been bench tested on both a Siyi ZR10 and Siyi A8. Tests included:

1. Using the MP Payload Control screen to pass roll, pitch and yaw angles to the vehicle. Out-of-range angles were sent but the gimbal only moved to its limits. Note: before and after tests were performed to confirm there was an issue before but not after.
2. Using MP's Data screen's, "Point Camera Here" feature the gimbal was pointed at a location, The vehicle was then rotated 360 degrees and I confirmed it yawed as closely as possible to the target but stayed within its body-frame limits.

The user discussion is here.

This has been successfully tested by KellyS on a real vehicle (facebook link)