kernel/net: honour the window scale factor advertised by the peer (#619)

Summary: the received window scale option was stored into SND_SCALE, which
the connection setup code then immediately overwrote with zero, so every
peer window was interpreted unscaled.

Details:
RFC 1323 negotiation is completed in two places -- the SYN_RECEIVED branch
and the active-open branch of tcp_input. Both do

        mov     ax, word[ebx + TCP_SOCKET.requested_s_scale]
        mov     word[ebx + TCP_SOCKET.SND_SCALE], ax

relying on the declared order of the four adjacent bytes SND_SCALE,
RCV_SCALE, requested_s_scale, request_r_scale to move both factors at once.
requested_s_scale, however, was never filled in: the option parser wrote the
peer's shift count into SND_SCALE directly, and that value was then clobbered
by the word move with the zero left in requested_s_scale by socket_alloc.

The result was that SND_SCALE ended up 0 on every connection while
TF_RCVD_SCALE was set, so a peer advertising, say, 64 KiB with a shift of 7
was read as advertising 512 bytes. Sending to any modern host was throttled
to a fraction of the real window.

The parser now stores into requested_s_scale, where the setup code expects
it, and clamps the value to TCP_max_winshift (14) as required by RFC 1323 --
a peer sending a larger shift must not be allowed to make our SND_WND
computation shift out of range.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Reviewed-on: #619
Reviewed-by: hidnplayr <hidnplayr@gmail.com>
Reviewed-by: Mikhail Frolov <mixa.frolov2003@gmail.com>
Co-authored-by: leency <lipatov.kiril@gmail.com>

kernel/trunk/network/tcp_input.inc

@@ -355,9 +355,14 @@ endl
         DEBUGF  DEBUG_NETWORK_VERBOSE, "TCP_input: Got window scale option\n"
         or      [ebx + TCP_SOCKET.t_flags], TF_RCVD_SCALE
 
+; remember the peer's scale factor; it is applied to SND_SCALE when the
+; connection is established (together with our RCV_SCALE)
         lodsb
-        mov     [ebx + TCP_SOCKET.SND_SCALE], al
-        ;;;;; TODO
+        cmp     al, TCP_max_winshift
+        jbe     .wscale_capped
+        mov     al, TCP_max_winshift
+  .wscale_capped:
+        mov     [ebx + TCP_SOCKET.requested_s_scale], al
 
        @@:
         jmp     .opt_loop