Ticket #2515, XCSoar not shutting down on some CE devices due to buggy serial-driver.
NEWS.txt: Added the changes made in 59b2e921aa3e6f8ce2226f80d52d1b415c40d61f
SerialPort: Code cleanup from 59b2e921aa3e6f8ce2226f80d52d1b415c40d61f (#2515).

Code verified to work on HP iPaq 314 (WM5).
Added ticket #2515 in NEWS.txt
TrailRenderer: Fixed potential unsigned underflow bug