Enable multiple defaults in multiple select widgets

This patch to select.inc enables you to enter multiple default values for select widgets that can have multiple values, like checkboxes. You enter the multiple values in the default field, seperated by commas. I also increased the allowed length of the default textfield to 256 since now you may have longer values needed here (as I found) and the db field is defined as text it requires no databases schema changes. A small bit of help text was added as well.