Allow APT_AUTH.CONF(5) config files to be used with archives during bootstrap.
Currently it's only supported in the chroot/binary phases. Omitting bootstrap was an oversight.
Note that config/archives/*.conf are assumed to be apt.conf files, so the apt_auth files are required to end with .auth and will be renamed to .conf when copied into the chroot.
See original MR: !260 (merged)