(jail -c -m src=/foo) cpdup can fail silently on automounted filesystems

[gpziemba]

The mount-point-crossover detection in cpdup gets confused when an automounted volume is automatically unmounted/remounted during a copy, causing the copy to be pruned without triggering an error exit code from cpdup. Poudriere has its own copy of cpdup that (as of poudriere 3.4.2 and today's date) is the same as DragonFlyBSD's cpdup.

I opened a DragonFlyBSD/cpdup PR with my proposed changes to cpdup. It does the right thing with automounts in my limited testing. Although I believe it to be correct, there isn't a regression test suite, so it's possible I overlooked something.

[bdrewery]

Nice. I'll keep an eye on that PR and bring the changes in after some review.