Added sendfile() support to socket streams

[agronholm]

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Changes

Closes #1259.

Checklist

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- Fix big bad boo-boo in task groups
  (`#123 <https://github.com/agronholm/anyio/issues/123>`_; PR by @yourgithubaccount)

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[agronholm]

This PR has...issues. I'll un-draft it when/if I've managed to solve them.

[agronholm]

One of the issues is that the fallback branch needs to call self.send() but that method grabs the send guard. The only solution is to make ResourceGuard re-entrant.