MAVLink SET_INTERVAL needs disable and default

The MAVLink msg [MAV_CMD_SET_MESSAGE_INTERVAL ](https://github.com/ArduPilot/mavlink/blob/master/message_definitions/v1.0/common.xml#L1481-L1485)says a value of 0us means default rate, and -1us means disable. These need to be added to the interval pulldown which is currently Hz only with 1 being the slowest. It would also be nice to be able to set that to a slower value. Generally, I don't like the pulldown method.


Start by locating the MAVLink message-interval pulldown shown in the issue and trace how its Hz values are converted into SET_MESSAGE_INTERVAL commands. Confirm the documented 0us default and -1us disable values, then define how a slower rate should be represented. Done means the interval control supports those values and a slower setting without breaking existing choices.

Written by the indexing model from the issue text.