ndb-dns-redistrib-size: one large UDP query kills the ndb/dns UDP server

sys/src/cmd/ndb/dnudpserver.c reads each datagram into a buffer of
Udphdrsize+Maxpayload bytes, then redistrib() copies it into a
static 1500-byte buffer behind

    assert(len <= sizeof outpkt);

A datagram with more than 1448 bytes of DNS payload fails the
assertion and the UDP server aborts.  It is forked RFNOWAIT and
nothing restarts it; TCP service carries on.  redistrib() runs
whether or not -T is given, so every ndb/dns -s is exposed.

The fix sizes outpkt like every other UDP buffer in ndb/dns.

Apply
    cd /
    ape/patch -p0 < /path/to/ndb-dns-redistrib-size/ndb-dns-redistrib-size.diff

Rebuild
    cd /sys/src/cmd/ndb && mk dns.safeinstall

    The old binary is kept as /$objtype/bin/ndb/odns.  Then restart
    ndb/dns; rebooting is simplest.

Verification
    python3 -c "import socket; socket.socket(socket.AF_INET,
        socket.SOCK_DGRAM).sendto(b'\0'*1600, ('<server>', 53))"
    dig SOA <your-zone> @<server>

    The datagram must leave in one piece; nc(1) may split its
    input into several smaller datagrams.

    Before: the console prints the assertion and dig times out.
    After: dig is answered.