--max-load doesn't check the variable

[Daniel Nichter]

Tool that have --max-load (and/or --critical-load) don't check if the variable actually exists. If no value is specified, then the tool (MySQLStatusWaiter.pm actually) will get the var's initial value, which will crash if the var doesn't exist. Else, when checking the var later, the tool just assumes the var is valid. This also depends somewhat on the get_status callback: it may crash too if given a bad/undefined var.

The tool should SHOW VARIABLES LIKE <var> when parsing the --max-load spec to ensure the var exists.