[ground_segment] represent unconfirmed settings with infinity

proposal to fix #1012:
- server: represent initial/unknown/unconfirmed settings with infinity
- page_settings: set label text to "?" upon commit and on update if value is "inf", also set label to "?"
[cockpit] add tooltip for current settings value label
[ground_segment] settings: also set current value lable to ? when requesting update
[ground_segment] use None/Some instead of abusing infinity
[settings] handle update from other agents than GCS and save based on last known value
Merge pull request #1023 from paparazzi/unconfirmed_settings

- add tooltip for current settings value label
- settings: set current value label to ? while unconfirmed (requesting update, commit)
- represent initial/unknown/unconfirmed settings with `None` in OCaml
- represent initial/unknown/unconfirmed settings with `?` in DL_VALUES on Ivy
- save based on last known value

solves #1012