udp: allow udp_tun_func_t() to indicate it did not eat the packet

[kp]

Summary

Allow udp tunnel functions to indicate they have not taken ownership of
the packet, and that normal UDP processing should continue.

This is especially useful for scenarios where the kernel has taken
ownership of a socket that was originally created by userspace. It
allows the tunnel function to pass through certain packets for userspace
processing.

The primary user of this is if_ovpn, when it receives messages from
unknown peers (which might be a new client).

Sponsored by: Rubicon Communications, LLC ("Netgate")