Make Unix and Windows impls consistent

There are some explicit Send/Sync implementations for Window's types
that don't exist in Unix. While the end result will be the same, I
believe it's clearer if we keep the explicit implementations consistent
by making the os-specific types Send/Sync where needed and possible.

This commit addresses tcp. Existing differences below:

src/libstd/sys/unix/tcp.rs
unsafe impl Sync for TcpListener {}
unsafe impl Sync for AcceptorInner {}

src/libstd/sys/windows/tcp.rs
unsafe impl Send for Event {}
unsafe impl Sync for Event {}
unsafe impl Send for TcpListener {}
unsafe impl Sync for TcpListener {}
unsafe impl Send for TcpAcceptor {}
unsafe impl Sync for TcpAcceptor {}
unsafe impl Send for AcceptorInner {}
unsafe impl Sync for AcceptorInner {}

Author: flaper87

src/libstd/sys/windows/tcp.rs

@@ -116,17 +116,13 @@ pub struct TcpAcceptor {
     deadline: u64,
 }
 
-unsafe impl Send for TcpAcceptor {}
-unsafe impl Sync for TcpAcceptor {}
-
 struct AcceptorInner {
     listener: TcpListener,
     abort: Event,
     accept: Event,
     closed: AtomicBool,
 }
 
-unsafe impl Send for AcceptorInner {}
 unsafe impl Sync for AcceptorInner {}
 
 impl TcpAcceptor {