plugin: sys_status: Move custom mode decoder to UAS.

Issue #136.
node: Implement reverse mode lookup.

Issue #136.
plugin: sys_status: Implement set_mode service.

Previous command shortcut removed.

Issue #136, #134.
scripts: Add mavsys tool.

Implented only `mode` operation.
Issue #134.
scripts: mavsys: Implement set rate command.