RFR: 8254854: [cgroups v1] Metric limits not properly detected on some join controller combinations

[Severin Gehwolf]

Originally only join controllers of form `cpu,cpuacct` would have been recognized.
However, this fails if more controllers are joined at a certain path.

The proposed fix is to be sure to split join controllers on `,` and set the path
accordingly. Otherwise, the Metrics system might be initialized due to some
non-join controllers, yet the limits for join controllers wouldn't be detected. A
test for this can be added once [JDK-8254001](https://bugs.openjdk.java.net/browse/JDK-8254001) has been implemented.

Testing:

- [x] jdk/submit
- [x] container tests on Linux cgroups v1 system.

Thoughts?

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Commit messages:
 - 8254854: [cgroups v1] Metric limits not properly detected on some join controller combinations

Changes: https://git.openjdk.java.net/jdk/pull/809/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=809&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254854
  Stats: 36 lines in 1 file changed: 3 ins; 10 del; 23 mod
  Patch: https://git.openjdk.java.net/jdk/pull/809.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/809/head:pull/809

PR: https://git.openjdk.java.net/jdk/pull/809