Fix ftbfs 20200718

collectd is scheduled for autoremoval tomorrow. These three patches get it building again (tested locally), by:

• disabling grpc and gmond, which are RC-buggy. They can be trivially re-enabled once they correct their bugs by reverting each patch.
• Patch the API to microhttpd. enum MHD_Result is the call signature for several functions, not int. This was easy, if tedious, to track down. Again, this can be reverted once upstream releases with a fix.

I'm going to submit a NMU upload to debian-mentors momentarily, emailing all the regular uploaders. I'd really like for this package to not get wiped out.