These work by keeping a set of all seen mroots as we descend down the
mroot chain. Simple, but it works.
The downside of this approach is that the mroot set grows unbounded, but
it's unlikely we'll ever have enough mroots in a system for this to
really matter.
This fixes scripts like dbgbmap.py getting stuck on intentional mroot
cycles created for testing. It's not a problem for a foreground script
to get stuck in an infinite loop, since you can just kill it, but a
background script getting stuck at 100% CPU is a bit more annoying.
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