Fix meterpreter not shutting down properly

The work that was done a while back to fix up command dispatching allowed
inline commands to run so that the server could be told to shutdown. Those
commands that want the server to terminate (such as migrate and shutdown)
should have returned `FALSE` instead of `TRUE` to tell the server thread to
stop.

I have no idea why those values were incorrect, but it's my work so it's
definitely my fault. I will have to sick back and lick my wounds for a while.
I hate it when I'm stupid.

Thanks to Kevin Mitnick for the bug, and @todb-r7 for the investigating the
history.

Redmine: [FixRM #8696]