Add constants for file system limits for Darwin

[jacob-carlborg]

Followup on #2460 for Darwin.

[dlang-bot]

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub fetch digger
dub run digger -- build "master + druntime#2462"

[jacob-carlborg]

thewilsonator approved these changes

That's like a record 😃.

[thewilsonator]

I try.