net: convert standalone IsSelectableSocket() to Sock::IsSelectable()

This makes the callers mockable.

src/net.cpp

@@ -961,8 +961,7 @@ void CConnman::CreateNodeFromAcceptedSocket(std::unique_ptr<Sock>&& sock,
         return;
     }
 
-    if (!IsSelectableSocket(sock->Get()))
-    {
+    if (!sock->IsSelectable()) {
         LogPrintf("connection from %s dropped: non-selectable socket\n", addr.ToString());
         return;
     }