allow negative marks for questions

In SAT and other tests, a wrong answer in a multiple choice question yields a deduction of 0.25 points (when given 5 choices). Even though Moodle let a teacher assign negative score to a choice, such a negative score is never recorded.

Suggested changes:

mod/quiz/locallib.php:867: change 0.0 to -1.0
mod/quiz/locallib.php:1787: comment out
mod/quiz/questiontypes/multichoice/questiontype.php:352: change 0.0 to -1.0