Send consistent SEG.WIN when using timewait codepath for TCP.
When sending TCP segments from the timewait code path, a stored
value of the last sent window is used. Use the same code for
computing this in the timewait code path as in the main code
path used in tcp_output() to avoid inconsistencies.
Fix a byte ordering issue for the advertised receiver window in ACK
segments sent in TIMEWAIT state, which I introduced in r336937.