Add option to ask users for personal information on first login.

Many users start sending out mail without a decent or even correct
"From" header. Defaults to on. Admin can disable if he has a
retrieve_user_info plugin or the guessed defaults are ok.

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@12274 7612ce4b-ef26-0410-bec9-ea0150e637f0