profiles/*/files: import what previously was 'configs' in configs (system) repository.

These are site-specific files that are likely to be modified by patch stage of
installer scripts.
functinos/runner: attempt to fix error handling madness.

We leverage traps that are set up in the runner core, so that we can remove
the ubiquitous `|| return 1` from most lines.
stage-pool/util-pacman-update@.sh: use call_pacman.
stage-pool/mnt-system-copy@.sh: use export_git and make use of traps in runner core to improve error handling.
profiles/__template: a profile template (with READMEs).
stage-pool/dest-install-files@.sh, profiles/*/stages: actually use the new infrastructure for copying files to target.