netdev-dummy: Add support for active stream

netdev-dummy: Add support for active stream

The dummy ports thus far only support passive connections. It can
listen for multiple incoming connection requests but not make active
connections. This patch adds support of active stream, so that a
dummy port can be configured with either passive or active connections.

The net result is that dummy ports can now connect to each other,
without being patch ports. This feature will be useful in adding test
cases of future commits.

Signed-off-by: Andy Zhou <azhou@nicira.com>