Also, please be informed that the Write method on both connected and
unconnected-mode sockets may return a positive number of bytes written
with timeout or use of closed network connection error.

Change-Id: I2e2e6192e29cef4e9389eb0422c605c6d12e6a3c
Reviewed-on: https://go-review.googlesource.com/9466Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>