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.

candm.h

@@ -243,6 +243,7 @@ typedef struct {
 #define REQ_ADDSRC_IBURST 0x4
 #define REQ_ADDSRC_PREFER 0x8
 #define REQ_ADDSRC_NOSELECT 0x10
+#define REQ_ADDSRC_TRUST 0x20
 
 typedef struct {
   IPAddr ip_addr;
@@ -478,6 +479,7 @@ typedef struct {
 
 #define RPY_SD_FLAG_NOSELECT 0x1
 #define RPY_SD_FLAG_PREFER 0x2
+#define RPY_SD_FLAG_TRUST 0x4
 
 typedef struct {
   IPAddr ip_addr;