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.