motor autosave: improve saving

Sometimes it seems a motor ioc can autosave invalid settings if there is an error in startup. There are some options we could look at to mitigate bad autosaves:
- use manual saves i.e. a "save" button needs to be pressed, or save command sent. We can run a background program a bit like autosave's `asVerify` to flag if current values differ from saved values so scientist can decide if they want to save.
- only enable autosave if the ioc thinks all is ok, this could be done by only calling `create_monitor_set` if all is known to be ok, or using `create_triggered_set` and only updating the trigger PV if all is known to be OK.