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.

memcached.c

@@ -222,7 +222,7 @@ static void settings_init(void) {
    settings.use_cas = true;
    settings.access = 0700;
    settings.port = 11211;
    settings.udpport = 11211;
    settings.udpport = 0;
    /* By default this string should be NULL for getaddrinfo() */
    settings.inter = NULL;
    settings.maxbytes = 64 * 1024 * 1024; /* default is 64MB */
@@ -7479,9 +7479,7 @@ int main (int argc, char **argv) {
        }
    }

    if (tcp_specified && settings.port != 0 && !udp_specified) {
        settings.udpport = settings.port;
    } else if (udp_specified && settings.udpport != 0 && !tcp_specified) {
    if (udp_specified && settings.udpport != 0 && !tcp_specified) {
        settings.port = settings.udpport;
    }

t/issue_67.t

@@ -77,11 +77,11 @@ sub when {

# Disabling the defaults since it conflicts with a running instance.
# when('no arguments', '', 11211, 11211);
when('specifying tcp port', '-p 11212', 11212, 11212);
when('specifying tcp port', '-p 11212', 11212, -1);
when('specifying udp port', '-U 11222', 11222, 11222);
when('specifying tcp ephemeral port', '-p -1', 0, 0);
when('specifying udp ephemeral port', '-U -1', 0, 0);
when('tcp port disabled', '-p 0', -1, 11211);
when('tcp port disabled', '-p 0', -1, -1);
when('udp port disabled', '-U 0', 11211, -1);
when('specifying tcp and udp ports', '-p 11232 -U 11233', 11232, 11233);
when('specifying tcp and disabling udp', '-p 11242 -U 0', 11242, -1);