Fixed tasyncawait on Windows.

Implicit registration of an fd now only occurs when a new socket is
created (in socket() or accept()). This makes the implementation much
simpler, changes to the linux version will follow.

lib/windows/winlean.nim

@@ -456,6 +456,7 @@ var
 
   SO_DONTLINGER* {.importc, header: "Winsock2.h".}: cint
   SO_EXCLUSIVEADDRUSE* {.importc, header: "Winsock2.h".}: cint # disallow local address reuse
+  SO_ERROR* {.importc, header: "Winsock2.h".}: cint
 
 proc `==`*(x, y: TSocketHandle): bool {.borrow.}