bsdinstall(8): kill dhclient before trying to get an IP

[will_worrbase.com]

Summary

kill dhclient before starting a new instance

Currently, if you're re-routed to the beginning of the install process or to the
beginning of the netconfig process, subsequent attempts to configure via
dhclient will fail as dhclient is already running.

Since there are many ways to get into this state, we should kill any dhclients
prior to configuring the primary interface with dhcp.

Test Plan

Tested by getting redirected to beginning of install process and
re-entering the netconfig portion