MixMonitor: Add class authorization requirements to MixMonitor AMI commands

MixMonitor AMI commands StartMixMonitor and StopMixMonitor lacked class
authorization. StopMixMonitor now requires that the manager user either have
the call or system class authorization. StartMixMonitor is a slightly larger
issue since it can execute shell commands if the right arguments are passed
into it, and we consider this a permission escalation. A security release
will be issued for problem this shortly.

ASTERISK-23609 #close
Reported by: Corey Farrell

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Merged revisions 415837 from http://svn.asterisk.org/svn/asterisk/branches/11



git-svn-id: https://origsvn.digium.com/svn/asterisk/certified/branches/11.6@415842 65c4cc65-6c06-0410-ace0-fbb531ad65f3