tcp: don't send data outside the remote window

[Dirbaio]

If we receive a packet with ack_seq=1000, win_len=10, this means the remote end is granting us permission to send bytes up to seq=1010 but no more.

The current implementation limits segment length to win, but does not limit the byte count. In the example above, it'll send many small packets: 1000-1010, 1010-1020 ... until the entire tx_buffer is sent.

This fixes the window handling so that only 1000-1010 is sent, and the rest isn't sent until the remote end sends a window update opening more space.

• Updated 1 test that tested for the previous behavior: test_established_send_buf_gt_win
• Updated a few tests that used the window to force sending small packets: they now use MSS.

[whitequark]

At a glance the change seems reasonable, but I haven't thought about it in-depth, so I primarily trust that you have tested it well.

[whitequark]

Thanks!