Implement handoff between DCB and FPS as source of W_PFSDSGN

We need to figure out how to decide whether the DCB or the FPS generates the W_PFSDSGN key. Obviously when the PFI is not on the telescope it should be the DCB, but it is less clear what should be done when the PFI is on.

Actually, I am not sure I know how to tell (without human intervention) whether the PFI is mounted. PEB power? AGCC link? pfiLamps? Something from the telescope?