Remove the 80% file system free space check

This check never worked as intended for several reasons:

- Didn't take the existing file size into account, causing it to
  succeed on first startup but fail on next

- Sparse files were not handled, and can't be handled as we don't know
  where in the file a hole could be and whether truncation actually
  would change the number of blocks used at all

Fixes: #1343
Don't allow relative sizes for stevedore file size
Test the return value more closely

Conflicts:
	bin/varnishtest/tests/m00018.vtc