socket: enable only specified IP families

Allow an IP family to be specified in the socket initialization in order
to globally disable the other family. This replaces the ntp_io and
cmdmon code handling the -4/-6 options and fixes a case where the NTP
client could still use a disabled family if the source was specified
with an IP address.

client.c

@@ -3601,7 +3601,7 @@ main(int argc, char **argv)
 
   UTI_SetQuitSignalsHandler(signal_handler, 0);
 
-  SCK_Initialise();
+  SCK_Initialise(IPADDR_UNSPEC);
   server_addresses = get_addresses(hostnames, port);
 
   if (!open_io())