sources: add require option

Require that at least one of the sources specified with this option is
selectable (i.e. recently reachable and not a falseticker) before
updating the clock. Together with the trust option this may be useful to
allow a trusted, but not very precise, reference clock or a trusted
authenticated NTP source to be safely combined with unauthenticated NTP
sources in order to improve the accuracy of the clock. They can be
selected and used for synchronization only if they agree with the
trusted and required source.

candm.h

@@ -244,6 +244,7 @@ typedef struct {
 #define REQ_ADDSRC_PREFER 0x8
 #define REQ_ADDSRC_NOSELECT 0x10
 #define REQ_ADDSRC_TRUST 0x20
+#define REQ_ADDSRC_REQUIRE 0x40
 
 typedef struct {
   IPAddr ip_addr;
@@ -480,6 +481,7 @@ typedef struct {
 #define RPY_SD_FLAG_NOSELECT 0x1
 #define RPY_SD_FLAG_PREFER 0x2
 #define RPY_SD_FLAG_TRUST 0x4
+#define RPY_SD_FLAG_REQUIRE 0x8
 
 typedef struct {
   IPAddr ip_addr;