[MEDIUM] stream_sock: don't close prematurely when nolinger is set

[MEDIUM] stream_sock: don't close prematurely when nolinger is set

When the nolinger option is used, we must not close too fast because
some data might be left unsent. Instead we must proceed with a normal
shutdown first, then a close. Also, we want to avoid merging FIN with
the last segment if nolinger is set, because if that one gets lost,
there is no chance for it to be retransmitted.