#7 Kill of system message tasks segfaults

[Ryan Daum]

When moving to new event code the check for
SYS_MESSAGE_EVENT in kill() was temporarily removed.
Killing a system message event (i.e. a PARSE, etc.)
causes a segfault.

[Ryan Daum]

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Made kill() a virtual method on Event, and overloaded
SysMessageEvent to forbid it.