[PATCH] Linux: Add execveat system call wrapper

[Florian Weimer]

* Joseph Myers:

> Note the need for a NEWS update for the new feature, and for documentation 
> of the new function in process.texi.

The manual parts are kind of my fault, I think they are blocked on a
generic section on *at functions and O_PATH.  I have posted a patch, but
the discussion was a bit involved, and got sidetracked afterwards.

Thanks,
Florian
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