Describe Your Changes

This PR was originally from #2010. I open this PR to merge the changes for subprocess functionality and keep #2010 smaller / more manageable. The improvements here will be used by my Python engine.

• Use cpp::result (instead of -1) to explicitly signal error when spawning process. This also addresses the problem that pid_t on Windows is DWORD i.e. unsigned int, which may lead to unexpected behavior if we use -1
• Add support for redirecting stdout and stderr when spawning subprocess
• On Windows, use a Job object to terminate the process and its children. Otherwise, children processes are not terminated.
• Create a ProcessInfo struct to clean up resources after process termination (right now only used by Windows to close handles)
• Fix IsProcessAlive() on Unix. kill(pid, 0) will return 0 (success) if the process has terminated but not reaped (i.e. exit status is still stored in process table). Hence, using waitpid(pid, NULL, WNOHANG) is more reliable.

Fixes Issues

• Closes bug: Issues with subprocess management in Windows #2036
• Closes idea: [subprocess] Redirect stdout and stderr of subprocess to file #2037

Self Checklist

• Added relevant comments, esp in complex areas
• Updated docs (for bug fixes / features)
• Created issues for follow-up changes or refactoring needed