Allow pubcookie module to alter the user_login form

This patch adds two new fields to the pubcookie settings page: "pubcookie_alter_login_form" which toggles _altering of the user_login form, and "pubcookie_authname" which replaces "pubcookie" in messages to the user (currently only in the altered login_form).

The login_form is altered to suggest that the user authentication via pubcookie.

This addresses the problem where "login or register to post comments" links still point to the local login form, and also might be useful for sites which allow both local and pubcookie authentication.