added "date of birth" as a users table field, added appropriate code to core

to get and set it, added "date" dialog field and updated registration,
profile, and admin modify to use it to change a user's date of birth; added
BDAY support to vCards and rigged user import/export to take it into account;
cleaned up a few other matters while I was in that particular code

setup/database.sql

@@ -90,6 +90,7 @@ CREATE TABLE users (
     lastaccess DATETIME,
     passreminder VARCHAR(255) DEFAULT '',
     description VARCHAR(255),
+    dob DATE,
     UNIQUE INDEX username_x (username)
 );