simulator: Fix argument parsing.

[teythoon]

When getopt_long sets *flag to val, it returns 0. Previously this was considered an error.

[Traumflug]

Thanks for the contribution!

I had a similar commit locally already (and forgot to push this), so I mixed these two and put it onto branch experimental: https://github.com/Traumflug/Teacup_Firmware/commit/7760ef3a345035d7bef2519c7a768610d6b93155