- credentials table includes did column

- authentication and reply sending decoupled, the new
  way to perform digest authentication is:
  if (!www_authenticate("@to.uri.host", "credentials")) {
      append_to_reply("%$digest_challenge");
      sl_reply("401", "Unauthorized");
      drop;
  }
- www_challenge and proxy_challenge deprecated (can be replaced
  with append_to_reply and sl_reply
- qop can be configured using modparam

db/schema/credentials.xml

@@ -39,6 +39,7 @@
 	<name>did</name>
 	<type>string</type>
 	<size>&id_len;</size>
+        <default>_none</default>
 	<description>
 	    DID of the virtual domain the username belongs to.
 	</description>

scripts/mysql/my_create.sql

@@ -68,7 +68,7 @@ CREATE TABLE missed_calls (
 INSERT INTO version (table_name, table_version) values ('credentials','7');
 CREATE TABLE credentials (
     auth_username VARCHAR(64) NOT NULL,
-    did VARCHAR(64) NOT NULL,
+    did VARCHAR(64) NOT NULL default '_none',
     realm VARCHAR(64) NOT NULL,
     password VARCHAR(28) NOT NULL DEFAULT '',
     flags INT NOT NULL DEFAULT '0',