Refine stream_select() to work with streams that have data in their read

Refine stream_select() to work with streams that have data in their read
buffers.
When selecting for read, the streams are examined; if any of them have
pending read data, no actual select(2) call is performed; instead the
streams with buffered data are returned; just like a regular select
call.
Prevent erroneous warning in stream_select when obtaining the fd.