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.
system: update checkout.

Submodule system f33acce..8d6e478:
  > framework: update checkout.
  > system/essential/verbatim-files: udev/rules.d: add 60-elevator.conf: default elevators for {,non}rotational devices.
  > system/essential/verbatim-files: issue, locale.conf: ditch, site-specific.
  > system/essential/verbatim-files: udev/rules.d/70-pm-usb-logitech.rules: ditch, site-specific.
  > configs: ditch.