[jdk17u-dev] RFR: 8298730: Refactor subsystem_file_line_contents and add docs and tests

[Goetz Lindenmaier]

I backport this for parity with 17.0.16-oracle.

I had to resolve one larger chunk in cgroupSubsystem_linux.hpp.  Changes [8301479: Replace NULL with nullptr in os/linux](https://github.com/openjdk/jdk21u-dev/commit/ac9e046748a9bb6ee065dc473d82135ce36043b7) and [8238161: use os::fopen in HS code where possible](https://github.com/openjdk/jdk21u-dev/commit/f54ce84474c2ced340c92564814fa5c221415944) are not in 17. 

I did add the new code as-is wrt. nullptr. but kept the call to fopen as it was in 17 before this change.

To make it compile I add changes to ostream.hpp from https://bugs.openjdk.org/browse/JDK-8293251: Use stringStream::base() instead of as_string() when applicable
See extra commit.

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Commit messages:
 - Parts of JDK-8293251
 - Backport 500c3c17379fe0a62d42ba31bdcdb584b1823f60

Changes: https://git.openjdk.org/jdk17u-dev/pull/3411/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=3411&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8298730
  Stats: 293 lines in 7 files changed: 232 ins; 15 del; 46 mod
  Patch: https://git.openjdk.org/jdk17u-dev/pull/3411.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev.git pull/3411/head:pull/3411

PR: https://git.openjdk.org/jdk17u-dev/pull/3411