Prevent a DoS by making the local interface non-blocking

This prevents a DoS from the local interface, which could be performed
by any user on the system running babeld.

A malicious attacker could connect to the local interface, and then
wait without reading any data.  When the send buffer eventually fills
up, babeld would block; as a result, it would become completely
inoperative.

Note that 'local_server_socket' is already non-blocking, but the
sockets spawned by accept() don't inherit this flag.  From accept(2):

  On Linux, the new socket returned by accept() does not inherit file
  status flags such as O_NONBLOCK and O_ASYNC from the listening socket.

babeld.c

@@ -827,6 +827,8 @@ main(int argc, char **argv)
 static int
 accept_local_connections(fd_set *readfds)
 {
+    int rc;
+
     if(local_server_socket < 0 || !FD_ISSET(local_server_socket, readfds))
         return 0;
 
@@ -849,6 +851,20 @@ accept_local_connections(fd_set *readfds)
         return -1;
     }
 
+    rc = fcntl(s, F_GETFL, 0);
+    if(rc < 0) {
+        fprintf(stderr, "Unable to get flags of local socket.\n");
+        close(s);
+        return -1;
+    }
+
+    rc = fcntl(s, F_SETFL, (rc | O_NONBLOCK));
+    if(rc < 0) {
+        fprintf(stderr, "Unable to set flags of local socket.\n");
+        close(s);
+        return -1;
+    }
+
     local_sockets[num_local_sockets++] = s;
     local_notify_all_1(s);
     return 1;