loop-util: let's cut trailing whitespace, not trailing lines

loop-util: let's cut trailing whitespace, not trailing lines

This doesn't really make any real difference, given the file should only
contain a single line. But it's conceptually more correct to just remove
the trailing newline/whitespace then the whole lines coming after that.
i.e. if the file actually contains more lines than one, this should
probably be considered an error.