functions/fs: add a distinct list_fs_registered function with correct semantics.
stage-pool/*: use list_fs_configured where applicable; get rid of kludges in functions/fs.
functions/runner: _prepare_stage_chroot: bail out if cannot unmount.

...avoid `rm -rf` at all cost.
functions/runner: _prepare_stage_chroot: bind top-level directories before the config directories.

Config directories now may lie below 'profiles'; while we could optimize by just
not mounting them, we need at least to respect topological ordering to allow the
whole bind tree to be unmounted depth-first.