Unify naming: all network stacks are in src/net_...

src/net_builtin.c

@@ -1,7 +1,6 @@
-#include "tcpip.h"
-
-#if MG_ENABLE_TCPIP
+#include "net_builtin.h"
 
+#if defined(MG_ENABLE_TCPIP) && MG_ENABLE_TCPIP
 #define MG_EPHEMERAL_PORT_BASE 32768
 #define PDIFF(a, b) ((size_t) (((char *) (b)) - ((char *) (a))))
 

src/net_builtin.h

@@ -1,10 +1,10 @@
 #pragma once
 
-#if MG_ENABLE_TCPIP
 #include "arch.h"
 #include "net.h"
 #include "queue.h"
 
+#if defined(MG_ENABLE_TCPIP) && MG_ENABLE_TCPIP
 struct mg_tcpip_if;  // Mongoose TCP/IP network interface
 
 struct mg_tcpip_driver {