Move N lines forward (backward if N is negative).
Precisely, if point is on line I, move to the start of line I + N
("start of line" in the logical order).
If there isn't room, go as far as possible (no error).
Returns the count of lines left to move. If moving forward,
that is N - number of lines moved; if backward, N + number moved.
With positive N, a non-empty line at the end counts as one line
successfully moved (for the return value). (fn &optional N)