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

[Severin Gehwolf]

Hi,

Please review this change which improves container detection support
tin the JVM. While docker container detection works quite well the
results for systemd slices with memory limits are mixed and depend on
the Linux kernel version in use. With newer kernel versions becoming
more widely used we should improve JVMs memory limit detection support
as well. This should be entirely backwards compatible as the
hierarchical limit will only be used if everything else is unlimited.

Bug: https://bugs.openjdk.java.net/browse/JDK-8217338
webrev: http://cr.openjdk.java.net/~sgehwolf/webrevs/JDK-8217338/04/webrev/

Testing: Manual testing of -XshowSettings and -Xlog:os+container=trace
with systemd slices on affected Linux distributions: Fedora 29,
recent Ubuntu (18-10). Existing docker tests pass. I'm currently also
running this through jdk/submit.

Thoughts?

Thanks,
Severin