Forms: Detect problem with PHP max_input_vars

1. Copy the temporary test file mdl41103.php from this issue into the root directory of your Moodle code tree. (NOTE: At time of writing it is also possible to test this issue using genuine forms such as the backup script, but I hope we will fix that later, so I don't want to put it in the test script. Anyway it's easier to use the test file.) 2. In your browser, visit /mdl41103.php?fields=50 3. You should see a form with 50 fields. Click the Submit button. EXPECTED: You should see the message 'Form submitted OK - max checkbox 50'. 4. Now visit /mdl41103.php?fields=1000 (this assumes you have not altered the max_input_vars setting in php.ini from the default; if you have altered it, use the same value as max_input_vars is set to) 5. Submit the form with 1000 checkboxes. EXPECTED: You should get a Moodle error message about incomplete form data. (Prior to this fix, step 5 would result in a supposedly 'correct' submission that only has 996 checkboxes.) Note: There is also an automated unit test as part of this code change.