BUG/MEDIUM: stream-int: Notify stream that the mux wants more room to xfer data

BUG/MEDIUM: stream-int: Notify stream that the mux wants more room to xfer data

When the mux failed to transfer data to the upper layer because of a lack of
room, it is important to wake the stream up to let it handle this
event. Otherwise, if the stream is waiting for more data, both the stream
and the mux reamin blocked waiting for each other.

When this happens, the mux set the CS_FL_WANT_ROOM flag on the
conn-stream. Thus, in si_cs_recv() we are able to detect this event. Today,
the stream-interface is blocked. But, it is not enough to wake the stream
up. To fix the bug, CF_READ_PARTIAL flag is extended to also handle cases
where a read exception occurred. This flag should idealy be renamed. But for
now, it is good enough. By setting this flag, we are sure the stream will be
woken up.

This patch is part of a series related to the issue #1362. It should be
backported as far as 2.0, probably with some adaptations. So be careful
during backports.