sources: enable selection options with authentication

When authentication is enabled for an NTP source, unauthenticated NTP
sources need to be disabled or limited in selection. That might be
difficult to do when the configuration comes from different sources
(e.g. networking scripts adding servers from DHCP).

Define four modes for the source selection to consider authentication:
require, prefer, mix, ignore. In different modes different selection
options (require, trust, noselect) are added to authenticated and
unauthenticated sources.

The mode can be selected by the authselectmode directive. The mix mode
is the default. The ignore mode enables the old behavior, where all
sources are used exactly as specified in the configuration.

sources.h

@@ -51,6 +51,14 @@ extern void SRC_Initialise(void);
 /* Finalisation function */
 extern void SRC_Finalise(void);
 
+/* Modes for selecting NTP sources based on their authentication status */
+typedef enum {
+  SRC_AUTHSELECT_IGNORE,
+  SRC_AUTHSELECT_MIX,
+  SRC_AUTHSELECT_PREFER,
+  SRC_AUTHSELECT_REQUIRE,
+} SRC_AuthSelectMode;
+
 typedef enum {
   SRC_NTP,                      /* NTP client/peer */
   SRC_REFCLOCK                  /* Rerefence clock */