[OPTIM] stream_sock: do not ask for polling on EAGAIN if we have read

[OPTIM] stream_sock: do not ask for polling on EAGAIN if we have read

It is not always wise to return 0 in stream_sock_read() upon EAGAIN,
because if we have read enough data, we should consider that enough
and try again later without polling in between.

We still make a difference between small reads and large reads though.
Small reads still lead to polling because we're sure that there's
nothing left in the system's buffers if we read less than one MSS.