functions.sh: add_config_directories_noruntime: fix a typo-ish mistake - iterate over arguments.
Revert "functions/fs: list_fs: list only configured filesystems."

This reverts commit bf048be21b43b1df53df1cf392cf0cc606836163.

list_fs is also used when configuring the filesystems; thus the semantic
change is incorrect.
functions/fs: add list_fs_registered function, properly checking whether fs has been configured.
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.