Refactor subsystem_file_line_contents and add docs and tests

The function subsystem_file_line_contents does a fairly 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.

Fix this by refactoring, adding tests, and documentation.