Fix DB differences in question->defaultmark and quiz_question_statistics->maxmark between installed & upgraded Moodle 2.1

Pasted from MDL-27929:

About question->defaultmark, it seems to be unsigned = true, both in install.xml and in its history in upgrade.php (comes from question->defaultgrade that always was unsigned). So I think we need to fix upgrade.php making it unsigned.

About quiz_question_statistics->maxmark, it seems to be unsigned = false in install.xml (and also another question_attempts->maxmark is unsigned=false). And in upgrade, we can find both situations, but @ 2008112102 the previous column (maxgrade) was changes explicity to unsigned = false. So I think we need to fix upgrade.php making it signed.

So, in summary, we need to add one more step to upgrade to:

1) modify question->defaultmark to unsigned = true:
2) modify quiz_question_statistics->maxmark to unsigned = false;