Add Process.environment as a [String:String] dictionary

[dunkelstern]

What's in this pull request?

Implemented Process.environment that takes it's values from the third parameter to the C main function (environ). Process.environment is a dictionary [String:String] of all defined environment variables. It does not update when the process executes a setenv system call.

Resolved bug number: (SR-1636)

Probably related: SR-1396

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[gribozavr]

Thank you for your contribution. Because this is an API change, it needs to be proposed through the Swift evolution process (https://github.com/apple/swift-evolution). I'm closing the pull request for now.

[jckarter]

Rather than propagate the poke-things-in-main hack in the compiler, we could just access the environ global directly from the standard library implementation.

[dunkelstern]

I don't think of it much a hack as it is just using the default way to get to that list but i can of course try to use environ but it seems it is not imported into swift so i would have to change some c++ code for that too.

[jckarter]

The way we set up argc and argv is broken, since it won't work for a DLL or a mixed-language project whose main isn't written in Swift. All of these globals really ought to be initialized from global state; unfortunately that requires unportable interfaces for argc and argv, but environ is supported on both BSD and Linux AFAICT.

[rintaro]

@dunkelstern FYI, https://bugs.swift.org/browse/SR-1119

[dunkelstern]

Ah now I understand, i'll rewrite the patch to use environ instead, but the way of importing that will be the same.