[misc] New net_peer_address_is_trusted with cfg set default to "lan"

New default for "trusted_networks" = "lan". This will check peer addresses
against the addresses/netmasks of the interfaces to establish whether the peer
is local.

Fixes #1754

src/httpd_artworkapi.c

@@ -150,7 +150,7 @@ artworkapi_request(struct httpd_request *hreq)
 {
   int status_code;
 
-  if (!httpd_admin_check_auth(hreq))
+  if (!httpd_request_is_authorized(hreq))
     return;
 
   if (!hreq->handler)