ntp: allow replacement of sources specified by IP address

For sources specified by an IP address, keep the original address as the
source's name and pass it to the NCR instance. Allow the sources to go
through the replacement process if their address has changed.

This will be useful with NTS-KE negotiation.

The IP-based source names are now provided via cmdmon. This means
chronyc -n and -N can show two different addresses for a source.

util.c

@@ -360,6 +360,16 @@ UTI_StringToIP(const char *addr, IPAddr *ip)
 
 /* ================================================== */
 
+int
+UTI_IsStringIP(const char *string)
+{
+  IPAddr ip;
+
+  return UTI_StringToIP(string, &ip);
+}
+
+/* ================================================== */
+
 int
 UTI_StringToIdIP(const char *addr, IPAddr *ip)
 {