Have gen2Actor archive based on mcs.mcsFileIds keyword

Per commissioning note about PFSC10789900 getting archived too early.

The gen2Actor currently listens to the mcs.filename keyword to trigger archiving. That is actually still generated a tiny bit early, in order to make part of fpsActor happy. There is an equivalent keyword mcs.mcsFileIds which really is only sent when it is fully safe to read the file.

Depending on which PFS host runs the program which the archiver gets files from, we might also want to add lookupcache=pos to that /etc/fstab mount line.