Fix MOBILE encapsulation. Add many debugging printfs (mainly
concerning UDP mode). Clean up the gre(4) code a bit. Add the
capability to setup UDP tunnels to ifconfig. Update documentation.
In UDP mode, gre(4) puts a GRE header onto transmitted packets,
and hands them to a UDP socket for transmission. That is, the
encapsulation looks like this: IP+UDP+GRE+encapsulated packet.
There are two ways to set up a UDP tunnel. One way is to tell the
source and destination IP+port to gre(4), and let gre(4) create
the socket. The other way to create a UDP tunnel is for userland
to "delegate" a UDP socket to the kernel.
47be766fd3