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

[Johan Sjölen]

> Hi!
> 
> Citing the ticket directly:
> 
> The function subsystem_file_line_contents does the simple job of parsing lines in a file, but does so in a fairly complex way. This function can be simplified. The function also has a surprising API, as you need to know that in one case the format string needs to take 2 specifiers, instead of 1. 
> 
> Some more context:
> 
> `subsystem_file_line_contents` either parses files that look like this:
> 
> 
> one_value
> 
> 
> Called as: `subsystem_file_line_contents(ctrl, fname, nullptr, "%s")`
> 
> Or like this:
> 
> 
> key1 val1
> key2 val2
> 
> 
> Called as:  `subsystem_file_line_contents(ctrl, fname, "key1", "%s %s")`
> 
> The API for the key/value case is changed to: `subsystem_file_line_contents(ctrl, fname, "key1", "%s")`. Note: `"%s"`, not `"%s %s"`.

Johan Sjölen has updated the pull request incrementally with one additional commit since the last revision:

  Add two more test cases

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/11667/files
  - new: https://git.openjdk.org/jdk/pull/11667/files/149646fe..6488af3b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11667&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11667&range=01-02

  Stats: 12 lines in 1 file changed: 12 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/11667.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11667/head:pull/11667

PR: https://git.openjdk.org/jdk/pull/11667