UserResetPasswordCommand now returns status code

The introduction of the command (see issue 0026632) broke the password
reset from the GUI (manage_user_reset.php), as some variables used in
the program were no longer initialized. As a result, it could only
unlock the account (i.e. reset the failed login count).

This commit changes the Command's process() method to return a status
code (RESET, UNLOCK or FAILURE, defined as class constants), which can
then be used by manage_user_reset.php to display an appropriate message.

Fixes 0026880