Add support for fragmenting ipv4 packets.
Add support for fragmenting ipv4 packets.

The packet filter may reassemble the ip fragments and return a packet that is
larger than the MTU of the sending interface. There is no check for DF or icmp
replies as we can only get a large packet to fragment by reassembling a
previous fragment, and this only happens after a call to pfil(9).

Obtained from:	OpenBSD (mostly)
Glanced at by:	mlaier
MFC after:	1 month

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