refactor: add parseCommandLine()

[WalterBright]

This just pulls the code that loops over arguments[] into parseCommandLine(). You can't really tell that from the github diff, as github gets thoroughly confused.

This makes the inputs and outputs for parsing the command line clear, and removes the global variable references from it. All instances of global.params in parseCommandLine() are replaced with just params. It's much better encapsulated.

More can be done, but this is a good first approximation.

[dlang-bot]

Thanks for your pull request, @WalterBright!

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[JinShil]

Since you're going through the work of refactoring this entire file, is it possible to start using D's string facilities instead of C's char * , strcmp, and friends?

[WalterBright]

Yes, but my main concern is better encapsulation, rather than a tangle of dependencies.

[JinShil]

If your main concern is a dependency on Phobos, I understand. But what about just druntime where string is defined. You can always add a string utilities module to DMD to avoid the dependency on Phobos.

[WalterBright]

By encapsulation I mean having clear inputs and outputs. Dependency on Phobos is a separate issue.

[MartinNowak]

LGTM