nts: close reusable sockets in helper process

Close all reusable sockets when the NTS-KE server helper is forked. It
is not supposed to have access to any of the server sockets, just the
socket for getting requests from the main process and the syslog socket.

nts_ke_server.c

@@ -685,6 +685,8 @@ run_helper(uid_t uid, gid_t gid, int scfilter_level)
 
   DEBUG_LOG("Helper started");
 
+  SCK_CloseReusableSockets();
+
   /* Suppress a log message about disabled clock control */
   log_severity = LOG_GetMinSeverity();
   LOG_SetMinSeverity(LOGS_ERR);