plugins/config: add 'nvme config status' command

[martin-belanger]

Add a command that reports on the legacy config files. It checks whether config.json/discovery.conf are present and whether they have already been converted to the new INI format (nvme-fabrics.conf), based on whether the rollback symlink is present next to each one. When a file has been converted, it hints that both the file and its symlink are safe to delete once rollback is no longer a concern.

[igaw]

Makes sense. The only thing I wonder if we should return an error code, so this command could be used in shell scripts without having to parse the output. The problem is no error is 0 and any error code is mapped to 1, because return codes in POSIX are a very difficult topic.

[martin-belanger]

I added a return code as follows:

EXIT STATUS
-----------
Returns 0 if neither legacy file has anything to report. Returns a
non-zero status if config.json or discovery.conf, or a '.converted'
marker for either, is present -- regardless of which of the states
listed above applies. Use the printed output, not the exit status, to
tell those states apart.

[igaw]

Thanks!