Implement IPMI support for RB_POWRECYCLE

Some BMCs support power cycling the chassis via the chassis control
command 2 subcommand 2 (ipmitool called it 'chassis power cycle'). If
the BMC supports the chassis device, register a shutdown_final handler
that sends the power cycle command if request and waits up to 10s for
it to take effect. To minimize stack strain, we preallocate a ipmi
request in the softc. At the moment, we're verbose about what we're
doing.

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