signup_status_schema_alter() is missing for the change to {signup_log}

While working on #581734-3: Improve the signup API: add hook_signup_data_alter(), hook_signup_create(), and hook_signup_update() I noticed that signup_status is not declaring its schema change of the {signup_log} table to add the 'status' column. That breaks things like drupal_write_record() working properly. Moving this bug to a separate issue.