Make std.socket DIP1000 compatible

[wilzbach]

From @WalterBright:

How about simply making std.socket @System so it will compile with -dip1000 and move on?

#6204 (comment)

[dlang-bot]

Thanks for your pull request, @wilzbach!

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[dnadlinger]

This comment seems odd/wrong – std.socket is just as safe as before, it's just that the send/receive arguments aren't marked as scope, so client code isn't safe anymore with dip1000.

[wilzbach]

Ok. Tried to improve the wording.

[dnadlinger]

Seems like the best compromise – -dip1000 might break some Phobos client code that previously escaped references to stack data this way, but we can't easily work around that.