.in: The FD must be readable; it becomes child's stdin.
.out: The FD must be writable; it becomes child's stdout.
- .err > 0 is not supported.
+ .err: The FD must be writable; it becomes child's stderr.
The specified FD is closed by start_command(), even if it fails to
run the sub-process!
* - Specify > 0 to set a channel to a particular FD as follows:
* .in: a readable FD, becomes child's stdin
* .out: a writable FD, becomes child's stdout/stderr
- * .err > 0 not supported
+ * .err: a writable FD, becomes child's stderr
* The specified FD is closed by start_command(), even in case
* of errors!
*/