MINOR: stconn: consider CF_SHUTW for sc_is_send_allowed()

MINOR: stconn: consider CF_SHUTW for sc_is_send_allowed()

When a shutdown(WR) is performed, send is no longer allowed, and that is
currently handled by the explicit cs_done_get() in the various shutw()
calls. That's a bit ugly and complicated for no reason, let's simply
integrate the test of SHUTW in sc_is_send_allowed().

Note that the test could also be added wherever sc_is_send_allowed() is
used but for now proceeding like this limits the changes.