inet_config: Tolerate atoms as `inetrc` kernel argument

[jhogberg]

Many RabbitMQ users provided atoms instead of strings as the inetrc file path because of an error in their documentation. This was tolerated by accident until atom filename support was removed from erl_prim_loader.

We did the latter for good reason, but tolerating this specifically in inet_config is no big deal.

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Fixes #8899. I figured that simply tolerating it was better than raising an error as it's difficult to present it in a way that makes sense, given how this argument is often provided on the command line.