This function almost writes itself. I need a thin layer to handle the
file IO errors and report them appropriately, and then all the magic is
a pass-through of the existing read_conf_str.
I've made basic unit tests for the most obvious scenarios. The test for
missing-file behavior is incomplete because I need to create a new error
variant.
I'm beginning work on the file reading functions, so I need some files
to read in my tests. I'll also need the WholeFile struct to compare
against.
The input string has been moved out into a file and put back into the
test fixture with `include_str!()`. The WholeFile construction has been
moved to a util function so I can reuse it in another test.
