AP_Follow: support for Mount following the lead vehicle in follow mode

[khanasif786]

While In follow mode, Mount also follows the same sysid the vehicle is following.
Tested in SITL.

[rmackay9]

Txs for this. It's looking pretty good, We should add a nullptr check of f and s.

[rmackay9]

actually it looks like the rest of ModeFollow::run() uses "g2.follow" so let's use that instead.

[peterbarker]

This doesn't look right. Mount can already follow a system ID - the only thing you might want to do is to add the option for follow mode to set the system ID for it to follow.

[rmackay9]

Ah, PeterB is right of course. This makes it much simpler, because the code can probably be added only to the init() method. Nice catch.

Another small thing is when we get the singleton we normally use a variable name similar to the library name. So instead of "AP_Mount *s" we might use "AP_Mount *mount".

[rmackay9]

.. and one last little thing is that PRs should always have a description even if the title makes it pretty obvious. .. normally that description should also include what testing has been done. If no testing has been done yet, then it's fine to say that and then we can edit the description later once it has been done.

[peterbarker]

Much nicer!

[peterbarker]

Ah. This PR needs splitting as it crosses libraries and ArduCopter directories.

Please use ./Tools/gittools/git-subsystems-split to make this multiple commits