ProcessBuilder: add method to use the sh wrapper

On Linux, processes built and executed with the Process component are
automatically wrapped by PHP inside a call to `sh`. This makes it
hard to use methods like `getPid()` or `stop()` since they work with
the `sh` process but not with the process the user intended to start.
The solution is to prepend the command being executed with
[exec](https://en.wikipedia.org/wiki/Exec_%28operating_system%29) (as
described in #5759). The `ProcessBuilder` now has two methods
`enableShellWrapper()` and `disableShellWrapper()` to enable or
disable the shell wrapper.