[link] improve datalink/telemetry report

- more reported data about up/down-link
- cleanup report messages
- display link page in GCS even in single link mode
[downlink] protect downlink rate computation
[link] fix link report
[link] handle correctly the fake link id (used in sim or while waiting for a new link)
Merge pull request #999 from paparazzi/link_report

[link] improve datalink/telemetry report

- more reported data about up/down-link
- cleanup report messages
- display link page in GCS even in single link mode

closes #442