BUG/MEDIUM: stream: Xfer the input buffer to a fully created stream

BUG/MEDIUM: stream: Xfer the input buffer to a fully created stream

The input buffer passed as argument to create a new stream must not be
transferred when the request channel is initialized because the channel
flags are not set at this stage. In addition, the API is a bit confusing
regarding the buffer owner when an error occurred. The caller remains the
owner, but reading the code it is not obvious.

So, first of all, to avoid any ambiguities, comments are added on the
calling chain to make it clear. The buffer owner is the caller if any error
occurred. And the ownership is transferred to the stream on success.

Then, to make things simple, the ownership is transferred at the end of
stream_new(), in case of success. And the input buffer is updated to point
on BUF_NULL. Thus, in all cases, if the caller try to release it calling
b_free() on it, it is not a problem. Of course, it remains the caller
responsibility to release it on error.

The patch fixes a bug introduced by the commit 26256f86e ("MINOR: stream:
Pass an optional input buffer when a stream is created"). No backport is
needed.