Duration form fields will accept fractional numbers of seconds, which cause DB errors

Steps to reproduce:

1. On a Moodle site using Postgres.
2. Create a quiz, and try to set the time limit to 9.3 seconds.

Actual result: when you try to save the form, you get a database error 'invalid input syntax for integer: "9.3"'.

Expected result: Either time is just saved as 9 seconds (I think this is the better option - anyone who thinks a web app working over the internet can be accurate to more than 1s is just wrong); or form validation error.