* Revert "Revert "Use Boost.Process instead of pid_t (#6510)" (#6909)"
This reverts commit bde575b8dd.
* Process wrapper, using Boost.Process on Windows
- Reverts bde575b8dd.
- Re-applies fb8e3615d5 after some refactoring.
* Remove Boost.Process dependency
* Don't open /proc file on Linux
* Change FATAL to ERROR and modify error message when process doesn't exist