include: clarify uv_tcp_bind() behavior

On BSD-like platforms, EADDRINUSE is returned by the bind() system call.
On other platforms, it's returned by the listen() system call.

In other words, some platforms are 'first to bind wins', others are
'first to listen wins' - but only with TCP sockets: UNIX domain sockets
always return EADDRINUSE from the bind() system call, UDP sockets don't
call listen() in the first place.

Fixes #769.