rather than explicitly running one helper after another in the major
build stages, or by hand (e.g. while testing things during development),
they can be run in bulk via this new helper. it essentially just takes a
list of helpers to run and runs them one by one.
it supports running all helpers except chroot_archives because that one
has different parameter requirements to the rest and supporting it would
make things messier.
helper scripts can either be named by their full script name or without
the 'chroot_' prefix for brevity. you can also just specify 'all' to
refer to all helpers (except chroot_archives, per above).
it automatically reverses the order of the list when run in remove mode.
Gbp-Dch: Short
e22bd27170