bug#45198: 28.0.50; Sandbox mode

[Mattias Engdegård]

Here is a bare-bones macOS sandbox implementation. In practice, it would 
probably be called in an --eval argument to guard anything executed later. It 
should be sufficient for the typical untrusted flymake checker running in an 
Emacs subprocess and printing to stdout/stderr.

macos-sandbox.diff
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