Custom Profile Field allow multiple selections in dropdown

[Mareid]

Currently the checkbox option and the select box option only allow a single entry.  I would like to allow multiple entries.  It may not be practical to have this show up in posts, but it would be very handy for administrators to know, for example the type of things a user is interested in

[Arantor]

This is unlikely to make an appearance in 2.1 (and I don't think it was already done and I just didn't notice), and fixing it for the future would be hard