tunneling: Factor out common UDP tunnel code.

Currently, the userspace VXLAN implementation contains the code
for generating and parsing both the UDP and VXLAN headers. This
pulls out the UDP portion for better layering and to make it
easier to support additional UDP based tunnels and features.

Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>