client.utils: Adds possibility to close descriptors before start process.

When autotest process don't close descriptors of stdin pipe before
child process is started then child process never die because
stdin pipe stays open by child process.

Signed-off-by: Jiří Župka <jzupka@redhat.com>
Merge pull request #820 from jzupka/fix-stdin-fd

client.utils: Adds possibility to close descriptors before start process...