[jdk8u-dev] RFR: 8284758: [linux] improve print_container_info

[Jonathan Dowland]

Low-risk, for Oracle parity. Mostly a clean-up. This will ease the future backporting of [JDK-8287011].

Doesn't apply clean: needs adjusting for paths and also context, to reflect 8u's lack of the pid controller ([JDK-8266490]).

[JDK-8287011]: https://bugs.openjdk.org/browse/JDK-8287011 (forthcoming)
[JDK-8287011]: https://bugs.openjdk.org/browse/JDK-8266490 (not considered)

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Depends on: https://git.openjdk.org/jdk8u-dev/pull/723

Commit messages:
 - 8284758: [linux] improve print_container_info

Changes: https://git.openjdk.org/jdk8u-dev/pull/724/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk8u-dev&pr=724&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8284758
  Stats: 52 lines in 1 file changed: 13 ins; 34 del; 5 mod
  Patch: https://git.openjdk.org/jdk8u-dev/pull/724.diff
  Fetch: git fetch https://git.openjdk.org/jdk8u-dev.git pull/724/head:pull/724

PR: https://git.openjdk.org/jdk8u-dev/pull/724