win: cause uv_read_stop to immediately stop reading the stream

this implements locking around the blocking call to ReadFile to get
around a Windows kernel bug where a blocking ReadFile operation on a
stream can deadlock the thread. this allows uv_read_stop to immediately
cancel a pending IO operation, and allows uv_pipe_getsockname to
"pause" any pending read (from libuv) while it retrieves the
sockname information

if unsupported by the OS (pre-Vista), this reverts to the old
(e.g. deadlock-prone) behavior

ref. issue #1313
No need, free(NULL) is guaranteed to be fine.