sources: add trust option

Assume time from a source that is specified with the trust option is
always true.  It can't be rejected as falseticker in the source
selection if sources that are specified without this option don't agree
with it.

srcparams.h

@@ -66,5 +66,6 @@ typedef struct {
 /* Flags for source selection */
 #define SRC_SELECT_NOSELECT 0x1
 #define SRC_SELECT_PREFER 0x2
+#define SRC_SELECT_TRUST 0x4
 
 #endif /* GOT_SRCPARAMS_H */