Implement vnet_null device

Such device don't need new threads and files but are valid ethernet
devices for the kernel. Convenient for testing.

vnet/null.c

@@ -0,0 +1,78 @@
+/*
+    SPDX-License-Identifier: GPL-2.0-or-later
+
+    UMKa - User-Mode KolibriOS developer tools
+    vnet - virtual network card, null interface
+
+    Copyright (C) 2023  Ivan Baravy <dunkaist@gmail.com>
+*/
+
+#include <fcntl.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+#include "trace.h"
+#include "umka.h"
+#include "vnet.h"
+
+static STDCALL void
+vnet_unload_null(void) {
+    COVERAGE_ON();
+    COVERAGE_OFF();
+}
+
+static STDCALL void
+vnet_reset_null(void) {
+    COVERAGE_ON();
+    COVERAGE_OFF();
+}
+
+/*
+static void
+dump_net_buff(net_buff_t *buf) {
+    for (size_t i = 0; i < buf->length; i++) {
+        printf("%2.2x ", buf->data[i]);
+    }
+    putchar('\n');
+}
+*/
+
+static STDCALL int
+vnet_transmit_null(net_buff_t *buf) {
+    struct vnet *net;
+    __asm__ __inline__ __volatile__ (
+        ""
+        : "=b"(net)
+        :
+        : "memory");
+
+//    dump_net_buff(buf);
+    buf->length = 0;
+    COVERAGE_OFF();
+    COVERAGE_ON();
+    return 0;
+}
+
+struct vnet *
+vnet_init_null(void) {
+    struct vnet *vnet = malloc(sizeof(struct vnet));
+    vnet->eth.net.device_type = NET_TYPE_ETH;
+    vnet->eth.net.mtu = 1514;
+    char *devname = malloc(8);
+    sprintf(devname, "UMKNUL%d", 0);   // FIXME: support more devices
+    vnet->eth.net.name = devname;
+
+    vnet->eth.net.unload = vnet_unload_null;
+    vnet->eth.net.reset = vnet_reset_null;
+    vnet->eth.net.transmit = vnet_transmit_null;
+
+    vnet->fdin = 0;
+    vnet->fdout = 0;
+    vnet->input_processed = 1;
+
+    memcpy(vnet->eth.mac, (uint8_t[]){0x80, 0x2b, 0xf9, 0x3b, 0x6c, 0xca},
+           sizeof(vnet->eth.mac));
+
+    return vnet;
+}

vnet/null.h

@@ -0,0 +1,16 @@
+/*
+    SPDX-License-Identifier: GPL-2.0-or-later
+
+    UMKa - User-Mode KolibriOS developer tools
+    vnet - virtual network card, null interface
+
+    Copyright (C) 2023  Ivan Baravy <dunkaist@gmail.com>
+*/
+
+#ifndef VNET_NULL_H_INCLUDED
+#define VNET_NULL_H_INCLUDED
+
+struct vnet *
+vnet_init_null(void);
+
+#endif  // VNET_NULL_H_INCLUDED