wiki.getAttachments returns 'perms' according to documentation

The xml-rpc methods [wiki.getAllPages] (https://www.dokuwiki.org/devel:xmlrpc#wikigetallpages), [wiki.getRecentMediaChanges] (https://www.dokuwiki.org/devel:xmlrpc#wikigetrecentmediachanges)  and [wiki.getAttachments] (https://www.dokuwiki.org/devel:xmlrpc#wikigetattachments) send information about permissions through a 'perms' attribute.

However, in practice, wiki.getAttachment doesn't but sends a 'perm' attribute (because of the way media are searched [1] (https://github.com/splitbrain/dokuwiki/blob/master/inc/search.php#L164)). It's therefore not consistent, and it doesn't respect Dokuwiki's documentation.

A solution could be the following patch:

    --- a/inc/search.php
    +++ b/inc/search.php
    @@ -161,8 +161,8 @@ function search_media(&$data,$base,$file,$type,$lvl,$opts){
         }
 
         //check ACL for namespace (we have no ACL for mediafiles)
    -    $info['perm'] = auth_quickaclcheck(getNS($info['id']).':*');
    -    if(!$opts['skipacl'] && $info['perm'] < AUTH_READ){
    +    $info['perms'] = auth_quickaclcheck(getNS($info['id']).':*');
    +    if(!$opts['skipacl'] && $info['perms'] < AUTH_READ){
             return false;
         }

There would be however two drawbacks:
* it would change an existing behavior (and therefore plugins relying on it)
* search_media would be inconsistent with search_universal

Hence this pull request which doesn't change search.php, but acts at the xml-rpc level.