* Add trailing newline on non-empty list in unlines
There are several reasons to do that:
* a line in a text file is something which ends with newline,
and the last line is not special
* `unlines []` should be different from `unlines [""]`
* `unlines (a ++ b) = unlines a ++ unlines b`
* Haskell does it
* Change lines function behaviour
