axfer: minor code arrangement for container module in a point of nonblocking flag

axfer: minor code arrangement for container module in a point of nonblocking flag

In internal container module, any file descriptor is expected as
non-blocking mode. Current implementation distinguish the case of
standard input and output from the case to open actual file since
O_NONBLOCK is used for the latter case. However, in both cases,
fcntl(2) is available to set non-blocking mode to the file descriptor.

This commit arranges to use fcntl(2) for both cases.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>