RFR: 8287007: [cgroups] Consistently use stringStream throughout parsing code

[Severin Gehwolf]

Please review this cleanup change in the cgroup subsystem which used to use hard-coded stack allocated
buffers for concatenating strings in memory. We can use `stringStream` instead which doesn't have the issue
of hard-coding maximum lengths (and related checks) and makes the code, thus, easier to read.

While at it, I've written basic `gtests` verifying current behaviour (and the same for the JDK code). From
a functionality point of view this is a no-op (modulo the one bug in the substring case which seems to be
there since day one). I'd prefer if we could defer any change in functionality to a separate bug as this is
meant to only use stringStream throughout the cgroups code.

Testing:
- [x] Container tests on Linux x86_64 cgroups v1 and cgroups v2
- [x] Added tests, which I've verified also pass before the stringStream change

Thoughts?

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

Commit messages:
 - Add cgroups v2 java test
 - use stringStream in cgroups v2
 - Add gtest for cgroups v2 code path
 - 8287007: [cgroups] Consistently use stringStream throughout parsing code
 - 8287007: [cgroups] Consistently use stringStream throughout parsing code

Changes: https://git.openjdk.java.net/jdk/pull/8969/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8969&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8287007
  Stats: 270 lines in 5 files changed: 235 ins; 19 del; 16 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8969.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8969/head:pull/8969

PR: https://git.openjdk.java.net/jdk/pull/8969