Synchronise snmp/mysql and agent-local/mysql; add debug CLI option

[jesusbagpuss]

Fixes #574 , which was already fixed in the snmp version, but not the agent version.

• Files have been synchronised as much as possible.
• Usage message has been updated with correct script name
• A --debug-log filename CLI option has been added to allow debug output without editing the script

NB These changes should also be accompanied with a change to the documentation https://docs.librenms.org/Extensions/Applications/MySQL/, in particular the following section:

Note also if you get a mysql error Uncaught TypeError: mysqli_num_rows(): Argument #1, this is because you are using a newer mysql version which doesnt support UNBLOCKING for slave statuses, so you need to also include the line $chk_options['slave'] = false; into mysql.cnf to skip checking slave statuses

[CLAassistant]

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