IANA (www.iana.org/assignments/service-names-port-numbers/
service-names-port-numbers.txt) has assigned an official port for
VXLAN (4789).  tcpdump uses this by default but we keep the -T
vxlan option as well (e.g. Linux is allowing different ports).
Merge pull request #310 from ffatghub/master

vxlan: use IANA assigned port