Revert "cgroups: only return path when subsystem really mounted

[crosbymichael]

We need to do this so that when a user specifies an option and we are unable to fulfill the request we should fail hard instead of ignoring the error and keep going. We also need to fail hard for cgroups like devices because of security reasons.

[LK4D4]

@crosbymichael first commit is not signed

[jessfraz]

Probably because it was done through the UI ;)

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#474 (comment).

[tianon]

Shouldn't these instead only fail hard like this if both the feature was requested and the join failed with NotFound?

Otherwise, any cgroup we add support for becomes required, like blkio did in #337 (and then revised via #437).

[jessfraz]

this is signed now :)

[LK4D4]

LGTM

[tianon]

c5eef90 thanks @crosbymichael ❤️

[hqhq]

Hi, the problem @tianon pointed out seems still exist. This revert also revert the fix that we don't fail when some subsystems are not mounted, I'm doing this another way in #476, so we also don't ignoring the error when a user specifies an option and we are unable to fulfill the request.