process: use io.Reader instead of io.WriteCloser for standard fds

Could someone explain why we should close this fds? Usually users
cares about closing them or not.
For example exec.Cmd declares them as io.Reader.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
libcontinaer: use new API in integration tests

Signed-off-by: Andrey Vagin <avagin@openvz.org>