Detect BATMAN interfaces, and treat them as wireless.

This makes Babel work (to the extent possible) over BATMAN interfaces, which
apparently some people are doing.  Thanks to Gioacchino Mazzurco and Sven
Eckelmann.

kernel_netlink.c

@@ -712,6 +712,25 @@ isbridge(const char *ifname, int ifindex)
     return 0;
 }
 
+static int
+isbatman(const char *ifname, int ifindex)
+{
+    char buf[256];
+    int rc;
+
+    rc = snprintf(buf, 256, "/sys/devices/virtual/net/%s/mesh", ifname);
+    if(rc < 0 || rc >= 256)
+        return -1;
+
+    if(access(buf, F_OK) >= 0)
+        return 1;
+
+    if(errno != ENOENT)
+        return -1;
+
+    return 0;
+}
+
 int
 kernel_interface_wireless(const char *ifname, int ifindex)
 {
@@ -721,10 +740,8 @@ kernel_interface_wireless(const char *ifname, int ifindex)
     struct ifreq req;
     int rc;
 
-    if(isbridge(ifname, ifindex) != 0) {
-        /* Give up. */
+    if(isbridge(ifname, ifindex) != 0 || isbatman(ifname, ifindex) != 0)
         return -1;
-    }
 
     memset(&req, 0, sizeof(req));
     strncpy(req.ifr_name, ifname, sizeof(req.ifr_name));