[hm] Only reduce, not fully cut, the I term when temperature is reached.

I'm seeing a lot of graphs where reaching the temperature cuts the PID
output so much that it can't recover before Lid Detect kicks in, turns
off the output and the cycle repeats. This change is a band-aid to that.
Failing not knowing how much I is needed, I'll take 1/4 as a starting
point. Experimentation may determing 1/2 or 3/4 might be even better.
[hm] Don't care about invalid results created by user entering 0mV/C for a thermocouple
[hm] Add GRILLPID_FAN_BOOST_ENABLED to momemtarily boost the fan when transitioning from 0% to "on"
[hm] Remove misleading comment

This comment has been wrong for like 2 years when the code to do that was
removed. Just goes to show, nobody looks at comments!
[hm] Don't store millis() in temp, use 16bit for elapsed time where they fit

Saves 46 bytes, mostly in tone_doWork()