Use net_close() to close sockets

So that it also works on Windows.

app/src/net.c

@@ -33,7 +33,7 @@ net_connect(uint32_t addr, uint16_t port) {
 
     if (connect(sock, (SOCKADDR *) &sin, sizeof(sin)) == SOCKET_ERROR) {
         perror("connect");
-        close(sock);
+        net_close(sock);
         return INVALID_SOCKET;
     }
 
@@ -61,13 +61,13 @@ net_listen(uint32_t addr, uint16_t port, int backlog) {
 
     if (bind(sock, (SOCKADDR *) &sin, sizeof(sin)) == SOCKET_ERROR) {
         perror("bind");
-        close(sock);
+        net_close(sock);
         return INVALID_SOCKET;
     }
 
     if (listen(sock, backlog) == SOCKET_ERROR) {
         perror("listen");
-        close(sock);
+        net_close(sock);
         return INVALID_SOCKET;
     }