properly unset alt_login field to avoid entry into data column of user table. better validation -- alt_login and username cannot be changed to be identical, and user cannot change username to an existing alt_login name.

#104167 -- alt login deleted when updating user profile. patch by hunmonk. only fire the update code if the update is coming from the save category.

New module.  Provides an interface that allows registered users to use a login name which is different than their username.