Experimental: On startup check wether our cert is seen as valid

This would be a better test than just wether we have previously
generated a cert. However, problems in need of investigation:

• This code seems to fail, if the system has several of our root-CAs installed, (see code comment).
• How to deal with use-case of remote proxy? Check for isLocalProxy()?
• Should have some UI support ("a cert is present, but does not seem to be accepted...")