Fix stdout premature EOF

Never close attached stream before both stdout and stderr have written
all their buffered contents. Remove stdinCloser because it is not needed
any more as the stream is closed anyway after attach has finished.

Fixes #3631

Signed-off-by: Andy Goldstein <agoldste@redhat.com>
Merge pull request #8571 from ncdc/3631-stdout-premature-eof

Fix stdout premature EOF