vconn: Fix tracking of "connected" state.

vconn: Fix tracking of "connected" state.

While I was looking at the rconn code for connection backoff and retry, I
noticed that ovs-vswitchd was logging the following on each connection
attempt:

   Jun 11 15:17:41|00020|vconn_stream|ERR|send: Connection refused

The "send:" part didn't make much sense.  The configured controller was not
actually running, so the vconn code should not have been able to connect
at all, so the message should have been about a connection failing, not
about sending on a completed connection failing.

Investigation showed that different parts of the library have different
ideas about return value semantics.  vconn_open() and stream_open() both
return 0 if a connection succeeded or if one is in progress, but some of
its callers thought that it returned 0 if the connection succeeded and
EAGAIN if the connection was in progress.  This commit fixes up the callers
that had the wrong idea, by making them instead all vconn_connect() or
stream_connect() to determine whether the connection is complete.