bsdinstall netconfig_ipv4: Fix resolv.conf rebuild

[asiciliano]

Summary

Trick to fix PR https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=262262.

Problem: After installation restart dhclient does not rebuild resolv.conf so "[bsd]dialog --mixedgaude" in bsdinstall/scripts/netconfig draws empty forms.
To reproduce:

1. FreeBSD-14 CURRENT
2. boot from the installer
3. begin installation
4. choose optional system components to install
5. configure networking (IPv4, DHCP, not IPv6)
6. observe the resolver configuration with automatically completed entries
7. choose [Cancel] without partitioning
8. choose [Restart]
9. .... Installation...
10. configure networking
11. PR: "resolver configuration is not automated"

To fix: just adding "ifconfig interface down" before to run dhclient.

Test Plan

I used mkisofs to rebuild an installation iso after the change, then testing the installer in VirtualBox.

I followed the handbook 18.5.3 https://docs.freebsd.org/en/books/handbook/book/#disks:

% mkisofs -R -no-emul-boot -b boot/cdboot -o /tmp/bootable.iso /tmp/myboot