std: Refactor process spawning on Unix

* Build up the argp/envp pointers while the `Command` is being constructed
  rather than only when `spawn` is called. This will allow better sharing of
  code between fork/exec paths.
* Rename `child_after_fork` to `exec` and have it only perform the exec half of
  the spawning. This also means the return type has changed to `io::Error`
  rather than `!` to represent errors that happen.

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