RFR: 8230848: OSContainer: Refactor container detection code

[Severin Gehwolf]

Hi,

Please review this code refactoring which will help getting a cgroups
v2 implementation for OSContainer implemented. The proposed changes are
mentioned in the bug. In essence, after this patch only methods
actually called in os_linux.cpp are exposed via OSContainer. Everything
else remains an implementation detail. After this refactoring it should
also be clearer what the actual bits implemented via cgroups v1 are in
hotspot. Functionally there should be no difference before and after
this patch.

If you are curious where I'm going with this, have a look at JDK-
8230305 which has a draft of an implementation of cgroups v2 building
on top of this patch.

Bug: https://bugs.openjdk.java.net/browse/JDK-8230848
webrev: http://cr.openjdk.java.net/~sgehwolf/webrevs/JDK-8230848/03/webrev/

Testing: jdk-submit, tier1 tests on Linux x86_64, container tests
         with docker and podman.

Thoughts?

Thanks,
Severin