Provide interface for external modules to set user subscriptions

This patch to mailchimp.module makes two changes -

1) mailchimp_user() loads the required lists against the user in hook_user(). It maintains the existing behaviour of setting lists according to the assigned roles.

function mailchimp_user($op, &$edit, &$account, $category = NULL) {
  // attach the required lists to the $account value
  if ($op == 'load') {
    if (!isset($account->mailchimp_lists)) {
      $required_lists = _mailchimp_get_required_lists();
      foreach ($required_lists as $key => $list) {
        if ($roles = array_intersect_key($account->roles, $list->roles)) {
          $account->mailchimp_lists[$key] = $list ;
        }
      }
    }
  } 
/* ... rest of mailchimp_user() here ... */ 

2) mailchimp_cron() checks $account->mailchimp_lists instead of $account->roles.

/* ... in mailchimp_cron() ... */
          foreach ((array)$lists as $key => $list) {
            if (array_intersect_key((array)$lists, $account->mailchimp_lists)) {
              $lists[$key]->batch[] = _mailchimp_load_user_list_mergevars($row->uid, $list->id, $lists[$key]->listMergeVars);
              $is_allowed = TRUE; // user is allowed in this list
            }
            else {
              $lists[$key]->unsubscribe[] = $account->mail;
            }              
          } 
/* ... in mailchimp_cron() ... */ 

(array_intersect_key() is PHP5, if mailchimp doesn't already require php5 I can provide an alternative).

By utilising the user object, we allow other modules to interact with the user's subscribed lists in hook_user(), and we can do things like this.

/**
 * Implements hook_user().
 */
function mymodule_user($op, &$edit, &$account, $category = NULL) {
  switch ( $op ) {
    case 'load' :
      if (isset($account->mailchimp_lists) && !isset($account->mailchimp_lists_done)) {
        $required = FALSE;
        if (!isset($account->profile)) {
          $account->profile = content_profile_load('profile', $account->uid, '', TRUE);
        }
        foreach (array('field_profile_a', 'field_profile_b') as $fieldname) {
          if (is_array($account->profile->$fieldname) && $account->profile->{$fieldname}[0]['value'] == 'Y') {
            $required = TRUE;
          }
        }
        if (!$required) {
          $account->mailchimp_lists = array();
        }
        $account->mailchimp_lists_done = TRUE;
      }
      break ;
  }
} 

The above code is part of an external module for a site which uses content_profile, and has fields "content_profile_a" and "content_profile_b".

Having "Y" for either of these fields means the site user should be included in required list subscriptions; otherwise they are unsubscribed.

We set those lists to "required" for 'authenticated user' role, then decide based on properties of the user (or their profile) whether the user will be subscribed or not.