Allow UDP packets to be fragmented

[Hanicef]

Experment to allow Overte to fragment packets, as it was explicitly disabling it before. Since it's known that some packets do go beyond MTU limits, those packets are guaranteed to be dropped in transit, so this might be a workaround for now until the packet sizes are fixed.

(cc @zedwick)

[daleglass]

This is likely to be more trouble than it's worth.

IP fragmentation results in fragments that lack the TCP/UDP headers. So a fragment doesn't have any port numbers, which firewalls really like to have.

I'm going to guess that this will cause some people a whole lot of trouble.

At the very least, implement it as a command-line option, so that people have to opt into it explicitly while troubleshooting. That way it's not going to break things for anyone that currently has no problems.

[ksuprynowicz]

I think this should be disabled by default and enabled by a command line parameter