dhclient: Add some regression tests

[markj]

Summary

Add tests for some recently fixed bugs in dhclient. All of these tests
set up two VNET jails connected by an epair. dhclient runs in one jail,
and the test code runs on the other end. It uses scapy to inject
"interesting" packets in response to DHCPDISCOVER messages from
dhclient, then checks the configured interface and lease file.