popen: allow to close parent's end of std* fds

The function popen_shutdown() checks whether std{in,out,err} was piped
and closes the parent's end. A user should have ability to send EOF for
child's stdin for stream programs like `grep`. It is better when there
is a function that encapsulates proper checks, error messages and the
actual actions.

This commit in particular reverts
1ef95b99 ('popen: remove redundant fd
check before perform IO'), because now the check is meaningful: an fd
may become closed before the whole popen handle will be deleted.

Part of #4031

Acked-by: Cyrill Gorcunov <gorcunov@gmail.com>