socket: add function to check supported family

Don't log error when an IPv6 socket cannot be opened and chronyd was
built without IPv6 support.

Reported-by: Lonnie Abelbeck <lonnie@abelbeck.com>

ntp_io.c

@@ -86,6 +86,9 @@ open_socket(int family, int local_port, int client_only, IPSockAddr *remote_addr
   int sock_fd, sock_flags, events = SCH_FILE_INPUT;
   IPSockAddr local_addr;
 
+  if (!SCK_IsFamilySupported(family))
+    return INVALID_SOCK_FD;
+
   if (!client_only)
     CNF_GetBindAddress(family, &local_addr.ip_addr);
   else