BUG/MEDIUM: stream_interface: don't close outgoing connections on shutw()

BUG/MEDIUM: stream_interface: don't close outgoing connections on shutw()

Commit 7bb68abb introduced the SI_FL_NOHALF flag in dev10. It is used
to automatically close the write side of a connection whose read side
is closed. But the patch also caused the opposite to happen, which is
that a simple shutw() call would immediately close the connection. This
is not desired because when using option abortonclose, we want to pass
the client's shutdown to the server which will decide what to do with
it. So let's avoid the close when SHUTR is not set.