Store all btrfs images in local subvolumes

Most btrfs images were stored in `_build`, but some were stored in the
yaml source directory.  Those in the yaml source directory are moved to
the yaml source directory subdirectory `_filesystem`.

The code that creates `_filesystem` creates a btrfs subvolume.  The code
to create the `_build` directory has been changed to create a subvolume
as well.

Using subvolumes for image file directories allows them to be excluded
from backup snapshots, which is highly desirable for these very large
files.

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