RFR: JDK-8273526: Extend the OSContainer API pids controller with pids.current [v3]

[Matthias Baesken]

> https://bugs.openjdk.java.net/browse/JDK-8266490
> extended the OSContainer API in order to also support the pids controller of cgroups. However only pids.max output was added with 8266490.
> There is a second parameter pids.current , see https://www.kernel.org/doc/html/latest/admin-guide/cgroup-v2.html#pid
> that would be helpful too and can be added to the OSContainer API .
> pids.current :
> A read-only single value file which exists on all cgroups.
> The number of processes currently in the cgroup and its descendants.
> 
> Best regards, Matthias

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

  Simplify coding following Severins advice

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/5437/files
  - new: https://git.openjdk.java.net/jdk/pull/5437/files/422aef68..afe0efd4

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=5437&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=5437&range=01-02

  Stats: 31 lines in 6 files changed: 1 ins; 27 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5437.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5437/head:pull/5437

PR: https://git.openjdk.java.net/jdk/pull/5437