tcp: Fix window scaling for accecpted socket

tcp: Fix window scaling for accecpted socket

- Retire tcpcb.requested_s_scale, use tcpcb.snd_scale directly.
- Set tcpcb.snd_wnd in SYN_SENT state only if the TCP flags contains SYN.
- Save other side advertised window into syncache, and setup tcpcb.snd_wnd
  according to the save value after the 3-way hand shake is done.
- Delay tiwin setup in tcp_input(), specificly after tcpcb.snd_scale is
  setup on the SO_ACCEPTCONN path.

This tends to fix the window scaling bug: when the sender accepts
connection and data only follow from sender to receiver.