Make it possible to provide a "Threads_running x 1.4" type option for the --max

The --critical-load option says: An optional "=MAX_VALUE" (or ":MAX_VALUE") can follow each variable. I'd like to see an extra option "x FACTOR" which would make the check relative to the checked value. This works better on a wider range of masters where the number of slaves or clients may vary considerably. So the option --critical-load="Threads_running x 1.4" would check the current value for this parameter and set the critical threshold as 1.4 x that value. The extra space makes reading the parameter easier. Whether that should be provided or not (or is optional) is less important, but allowing it for readability is good.