RFR: JDK-8266490: Extend the OSContainer API to support the pids controller of cgroups [v3]

[Matthias Baesken]

> Hello, please review this PR; it extend the OSContainer API in order to also support the pids controller of cgroups.
> 
> I noticed that unlike the other controllers "cpu", "cpuset", "cpuacct", "memory"  on some older Linux distros (SLES 12.1, RHEL 7.1) the pids controller might not be there (or not fully supported) so it was added as optional  , see the coding
> 
> 
>   if (!cg_infos[PIDS_IDX]._data_complete) {
>     log_debug(os, container)("Optional cgroup v1 pids subsystem not found");
>     // keep the other controller info, pids is optional
>   }

Matthias Baesken has updated the pull request incrementally with one additional commit since the last revision:

  test and small adjustments suggested by Severin

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/4518/files
  - new: https://git.openjdk.java.net/jdk/pull/4518/files/afd7bf61..f5527143

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4518&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4518&range=01-02

  Stats: 103 lines in 7 files changed: 92 ins; 2 del; 9 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4518.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4518/head:pull/4518

PR: https://git.openjdk.java.net/jdk/pull/4518