[MINOR] stream_sock: always clear BF_EXPECT_MORE upon complete transfer

[MINOR] stream_sock: always clear BF_EXPECT_MORE upon complete transfer

When sending is complete, it's preferred to systematically clear the flags
that were set for that transfer. What could happen is that the to_forward
counter had caused the MSG_MORE flag to be set and BF_EXPECT_MORE not to
be cleared, resulting in this flag being unexpectedly maintained for next
round.

The code has taken extreme care of not doing this till now, but it's not
acceptable that the caller has to know these precise semantics. So let's
unconditionnally clear the flag instead.

For the sake of safety, this fix should be backported to 1.4.