Added 'ip fou...' commands to enable/disable UDP ports for doing
foo-over-udp and Generic UDP Encapsulation variant. Arguments are port
number to bind to and IP protocol to map to port (for direct FOU).

Examples:

ip fou add port 7777 gue
ip fou add port 8888 ipproto 4

The first command creates a GUE port, the second creates a direct FOU
port for IPIP (receive payload is a assumed to be an IPv4 packet).
Signed-off-by: Tom Herbert <therbert@google.com>