Downport 8217338: [Containers] Improve systemd slice memory limit support

[Lindenmaier, Goetz]

Hi Severin, 

I'm working on downporting the 11.0.7-oracle changes.
https://bugs.openjdk.java.net/browse/JDK-8232207: Linux os::available_memory re-reads cgroup configuration on every invocation
depends on 8217338.  You had once intended to downport 
https://bugs.openjdk.java.net/browse/JDK-8217338.
The downport was then abandoned.

I am wondering whether you want to pick this up again?
8217338 is productive in 13 for a while now, and it applies
clean.
I'm not sure about the Java changes, do they change the API?
The hotspot changes help to apply 8232207, though.

I would like to wait with 8232207 until this is decided.

Best regards,
  Goetz.