PMAC: Start up an IOC

Part of #2
An initial target of the PMAC simulation is to have a ticket device that can trick EPICS into thinking it is connected to a real PMAC to the extent that an IOC can be started against it. For this, no simulation logic is needed, we just need to have the device respond to IOC start up/status commands in such a way that the IOC doesn't immediately break.
This will require https://github.com/dls-controls/tickit/pull/80 to strip off the 8 byte headers from EPICS PMAC commands within the message interpreter. EPICS sends multiple commands in a single message, so https://github.com/dls-controls/tickit/pull/82 will also be needed to split up each message into the individual commands within the device's message interpreter.