RFR: JDK-8266490: Extend the OSContainer API to support the pids controller of cgroups [v7]

[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 with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains seven additional commits since the last revision:

 - Merge remote-tracking branch 'origin/master' into JDK-8266490
 - Minor adjustments, handling of Unlimited
 - Merge remote-tracking branch 'origin/master' into JDK-8266490
 - Add hotspot tests
 - test and small adjustments suggested by Severin
 - Adjustments following Severins comments
 - JDK-8266490

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/4518/files
  - new: https://git.openjdk.java.net/jdk/pull/4518/files/857ab1db..39b50089

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4518&range=06
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4518&range=05-06

  Stats: 4915 lines in 198 files changed: 3503 ins; 507 del; 905 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