ipfw: Update warning/error setsockopt references

Dummynet v3 switched to IP_DUMMYNET3 but did not update these
warnings/errors.

Fixes: cc4d3c30ea28 ("Bring in the most recent version of ipfw and dummynet, developed")
Sponsored by: The FreeBSD Foundation
Differential Revision: sbin/ipfw/ipfw2.c