Create consistent /etc/hosts in build chroots with USENETWORK=no

Policy 4.2.0.0 §4.9 explicitly allows loopback access.
We already set up the build chroot in a way that ifconfig shows:

lo: flags=73<UP,LOOPBACK,RUNNING>  mtu 65536
        inet 127.0.0.1  netmask 255.0.0.0
        inet6 ::1  prefixlen 128  scopeid 0x10<host>
        loop  txqueuelen 1000  (Local Loopback)
        RX packets 0  bytes 0 (0.0 B)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 0  bytes 0 (0.0 B)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

We already also clean resolv.conf; add a clean hosts file to that
which ships 127.0.0.1 as localhost and ::1 as localhost6 to avoid
trouble. (It also ships a couple other standard IPv6-related en‐
tries; these are optional, but don’t hurt.)

This patch also adds rm statements to make sure hardlinks on the
replaced files are broken up.

Closes: #905307
Signed-off-by: mirabilos <mirabilos@evolvis.org>
Signed-off-by: Mattia Rizzolo <mattia@debian.org>