Have the gen2.instIds Instrument be dynamic

The Gen2 inst_ids=NAOJ,Subaru,PFS key is actually just static strings. The instrument part should track the active main instrument. This would be useful now for two purposes: the let SUNSS know the actual instrument name, and to let us block MLP1 traffic when PFS is not the main instrument.

The Gen2 FITS.SBR.MAINOBCP key shows the currently allocated instrument and we can use that. To be a bit more careful (for MLP1) we can double-check by looking at STATL.M2_DESCR which will be "PFS at M2" if and only if PFS is mounted at prime focus. [ per @ej ]