Making workaround for apt-get source actually working in backports-included-but-not-enabled scenarios.

Otherwise users opting for XFS and JFS filesystems and installing without
network won't have them and the (first) boot can be interrupted due to
this.

Sponsored-by: Offensive Security

Stripping backports from sources.list temporarily when obtaining sources to workaround apts disregard of pinning for the cases where backports are included but not enabled.

Expanding all variables in all syslinux configuration files, rathern than on some selected ones only.

Moving caching out of bootstrap_{c,}debootstrap to own helper for future unification with chroot caches helper.

Handling dist-upgrade right after bootstrap with copy of chroot_archives as bootstrap_archives in preparation for a future multi-archive bootstrap.

It is important for derivatives that the bootstrap cache is setup completely,
it is re-used in several places and thus cannot consist of the parent
distribution only.

Until there's a supportable way to bootstrap from multiple archives at the same time
(and multistrap doesn't work for that as discussed in the past), we're going to run
a lot of duplicated code here to massage the apt sources for the time being.

Dropping global archive definitions shipped in /usr/share/live/build/archives (same as with packagelists, hidden archive 'magic' is gone now in favour of explicit config tree configuration).

This reverts commit c944301b.