Don't touch correct target xattrs; remove inappropriate target xattrs.

Don't touch correct target xattrs; remove inappropriate target xattrs.

When applying xattr metadata, make sure to remove any target xattrs
that aren't in the metadata record, but don't touch target xattrs that
already match the metadata record.  Add corresponding tests.

Throw an ApplyError() if xattr set() or remove() return EPERM from
within _apply_linux_xattr_rec().

Remove lib/bup/t/testfs and lib/bup/t/testfs.img in the clean target.