one or two file descriptors for communication with the subprocess.
If the subprocess creation fails: if exit_on_error is true, the main
process exits with an error message; otherwise, an error message is given
- if null_stderr is false, then -1 is returned and fd[] remain uninitialized.
+ if null_stderr is false, then -1 is returned, with errno set, and fd[]
+ remain uninitialized.
After finishing communication, the caller should call wait_subprocess()
to get rid of the subprocess in the process table.