Password expiration date

Description

It would be nice if it would be possible to set an expiration date for a user's password so that they'd have to change it after a set period of time (or date).

[jolheiser]

This doesn't directly implement the requested feature, but I thought I'd post it here so people can use it as a temporary fix with 1.8+

#6193 will add the MustChangePassword options to the API for creating/editing a user, so this could viably be done with a cron job until the feature is potentially implemented.