libct/cg: skip hidden ones when parsing cgroup mounts

[liusdu]

According to [1], "If a new mount is stacked on top of a previous existing mount, at pathname P, then the parent of the new mount is the previous mount at that location"

Use this strategy to skip hidden cgroup mount.

1. https://man7.org/linux/man-pages/man5/proc.5.html

[kolyshkin]

@liusdu can you create an integration test case, so that we can see the problem the code is trying to solve?

[lifubang]

@liusdu I mark this pr as draft. Please rebase, fix the errors, and add some test cases.

[kolyshkin]

@liusdu do you plan to keep working on this?