Allow an MTU of up to 65535 bytes on tun interfaces

[tuexen]

Summary

Currently tun interfaces can only receive packets up to 16384. To bump this to 65535, the struct tuninfo also needs to be changed. The mtu component is of type short and needs to be changed to unsigned short to allow for values up to 65535.

Using MTUs for up to 65535 bytes is useful for testing transport stacks with packetdrill for interfaces with large MTUs.

Test Plan

The patch has been tested with packetdrill and an MTU of 65535 and the following test scripts:
https://github.com/sctplab/sctp-idata/blob/master/tests/sctp-idata-92.pkt
https://github.com/sctplab/sctp-idata/blob/master/tests/sctp-idata-93.pkt

[bz]

Seems usr.sbin/ppp is the only in-tree consumer and they assign from an unsigned long;
You might want to go and check openvpn and possibly mpd; not sure what other ports; hercules or qemu maybe?
Worst ask portmgr to do an exp run via bugs.

[tuexen]

I'll look at some of the ports. I don't think the change will be a problem, but I'll double check.

[tuexen]

I checked the ports, couldn't find any issues. Thanks for the suggestion. Will commit the patch.

[tuexen]

This patch was committed in https://svnweb.freebsd.org/changeset/base/300603.