conf: add ipv4 and ipv6 options to server/pool/peer directive

Accept "ipv4" and "ipv6" options in the server/pool/peer directive to
use only IPv4 or IPv6 addresses respectively.

The configuration is different from the "server [-4|-6] hostname" syntax
supported by ntpd to avoid breaking existing scripts which expect the
hostname to always be the first argument of the directives.

cmdparse.h

@@ -32,6 +32,7 @@
 
 typedef struct {
   char *name;
+  int family;
   int port;
   SourceParameters params;
 } CPS_NTP_Source;