vmount: properly unadvertise uorb topics when stopping or re-configuring vmount. Othwerise subsequent advertise calls may fail (only ever witnessed for _mount_orientation_pub)

control_data needs to be able to be set to nullptr in case if the input
is already active. Otherwise the output will think there's always new
requests and reset it's state.

This avoids a busy-loop if the input is listening for vehicle commands and
the output publishes vehicle commands.

Issue: This conversion test failed with gcc 7.1.1 and clang 4.0.1:
```
ERROR [tests] conversion fail: input:  -0.8188, intermediate: -8187, result:  -0.8187
conv FAILED
Command 'tests' failed, returned -1
```

Fix: explicit rounding

-closes #7539

... and some strange reformatting due to astyle...

This will initialize those structs with zero in all fields not set
and all fields set will only be change once to the final value not
wasting CPU time zeroing it.

This will guarantee that no non-unitialized structs will have
a trash value on from_external causing it to be sent to the
MAVLink channel without need it.

The previous approach was checking system id and component id but it
will not work in 100% of cases as external devices can send MAVLink
message with the right system id but with broadcast component id.

No need to keep this struct as global or alive while looping.

Lets save a few bytes using the right data types.