Make upgrade and addnotes idempotent

[wolfgangwalther]

This allows running the setnotes.pl and update_personality.pl scripts in the client-code repo repeatedly without causing multiple changes / rows on the server-side.

This has the following advantages:

• It fixes a theoretical problem when the client sends the request successfully, but doesn't receive the response due to a network error - and then tries the same thing again, ending up with a duplicate row for the os/compiler version.
• It allows automating the update of os / compiler versions (and notes, too, in case they contain any version information). This can be as simple as a script run via the same cron job as the build script itself - checking for the current os and compiler version and sending those before each run.