bsdinstall netconfig_ipv4: Fix resolv.conf rebuild

After an installation restart (for error or choice) dhclient does not
rebuild resolv.conf so `dialog --mixedform' of "Resolver Configuration"
in bsdinstall/scripts/netconfig draws empty forms. It causes a bad UX,
to see PR262262. Fixed resetting the interface before to run dhclient.

PR: 262262
Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D35094