disable UDP port by default

As reported, UDP amplification attacks have started to use insecure
internet-exposed memcached instances. UDP used to be a lot more popular as a
transport for memcached many years ago, but I'm not aware of many recent
users.

Ten years ago, the TCP connection overhead from many clients was relatively
high (dozens or hundreds per client server), but these days many clients are
batched, or user fewer processes, or simply anre't worried about it.

While changing the default to listen on localhost only would also help, the
true culprit is UDP. There are many more use cases for using memcached over
the network than there are for using the UDP protocol.
static void clock_handler(const int fd, const short which, void *arg) {
    struct timeval t = {.tv_sec = 1, .tv_usec = 0};
    static bool initialized = false;
#if defined(HAVE_CLOCK_GETTIME) && defined(CLOCK_MONOTONIC)
    static bool monotonic = false;
    static time_t monotonic_start;
#endif

    if (initialized) {
        /* only delete the event if it's actually there. */
        evtimer_del(&clockevent);
    } else {
        initialized = true;
        /* process_started is initialized to time() - 2. We initialize to 1 so
         * flush_all won't underflow during tests. */
#if defined(HAVE_CLOCK_GETTIME) && defined(CLOCK_MONOTONIC)
        struct timespec ts;
        if (clock_gettime(CLOCK_MONOTONIC, &ts) == 0) {
            monotonic = true;
            monotonic_start = ts.tv_sec - ITEM_UPDATE_INTERVAL - 2;
        }
#endif
    }

    assoc_start_expand(stats_state.curr_items);

    evtimer_set(&clockevent, clock_handler, 0);
    event_base_set(main_base, &clockevent);
    evtimer_add(&clockevent, &t);

#if defined(HAVE_CLOCK_GETTIME) && defined(CLOCK_MONOTONIC)
    if (monotonic) {
        struct timespec ts;
        if (clock_gettime(CLOCK_MONOTONIC, &ts) == -1)
            return;
        current_time = (rel_time_t) (ts.tv_sec - monotonic_start);
        return;
    }
#endif
    {
        struct timeval tv;
        gettimeofday(&tv, NULL);
        current_time = (rel_time_t) (tv.tv_sec - process_started);
    }
}

static void clock_handler(const int fd, const short which, void *arg) {
    struct timeval t = {.tv_sec = 1, .tv_usec = 0};
    static bool initialized = false;
#if defined(HAVE_CLOCK_GETTIME) && defined(CLOCK_MONOTONIC)
    static bool monotonic = false;
    static time_t monotonic_start;
#endif

    if (initialized) {
        /* only delete the event if it's actually there. */
        evtimer_del(&clockevent);
    } else {
        initialized = true;
        /* process_started is initialized to time() - 2. We initialize to 1 so
         * flush_all won't underflow during tests. */
#if defined(HAVE_CLOCK_GETTIME) && defined(CLOCK_MONOTONIC)
        struct timespec ts;
        if (clock_gettime(CLOCK_MONOTONIC, &ts) == 0) {
            monotonic = true;
            monotonic_start = ts.tv_sec - ITEM_UPDATE_INTERVAL - 2;
        }
#endif
    }

    assoc_start_expand(stats_state.curr_items);

    evtimer_set(&clockevent, clock_handler, 0);
    event_base_set(main_base, &clockevent);
    evtimer_add(&clockevent, &t);

#if defined(HAVE_CLOCK_GETTIME) && defined(CLOCK_MONOTONIC)
    if (monotonic) {
        struct timespec ts;
        if (clock_gettime(CLOCK_MONOTONIC, &ts) == -1)
            return;
        current_time = (rel_time_t) (ts.tv_sec - monotonic_start);
        return;
    }
#endif
    {
        struct timeval tv;
        gettimeofday(&tv, NULL);
        current_time = (rel_time_t) (tv.tv_sec - process_started);
    }
}
