auth: added new error code to auth API

- AUTH_USER_MISMATCH = -8 -- to be returned when auth user mistmach
  from/to header user

modules/auth/api.h

@@ -44,6 +44,7 @@
  * return codes to config by auth functions
  */
 typedef enum auth_cfg_result {
+	AUTH_USER_MISMATCH = -8,    /*!< Auth user != From/To user */
 	AUTH_NONCE_REUSED = -6,     /*!< Returned if nonce is used more than once */
 	AUTH_NO_CREDENTIALS = -5,   /*!< Credentials missing */
 	AUTH_STALE_NONCE = -4,      /*!< Stale nonce */