[jdk11u-dev] RFR: 8298730: Refactor subsystem_file_line_contents and add docs and tests [v3]

[Jonathan Dowland]

> Not clean, adjustments necessary to adapt for pre-C++-11, and a small change to account for lack of JEP 371.

Jonathan Dowland has updated the pull request incrementally with one additional commit since the last revision:

  Update test/hotspot/gtest/os/linux/test_cgroupSubsystem_linux.cpp
  
  Co-authored-by: Severin Gehwolf <jerboaa at gmail.com>

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

Changes:
  - all: https://git.openjdk.org/jdk11u-dev/pull/3149/files
  - new: https://git.openjdk.org/jdk11u-dev/pull/3149/files/39940791..8564738f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=3149&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=3149&range=01-02

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/3149.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev.git pull/3149/head:pull/3149

PR: https://git.openjdk.org/jdk11u-dev/pull/3149