new option max-children that limits the number of concurrent child processes

xiosigchld.c

@@ -104,6 +104,7 @@ void childdied(int signum) {
 	 return;
       }
    /*! indent */
+   if (num_child) num_child--;
    /* check if it was a registered child process */
    i = 0;
    while (i < XIO_MAXSOCK) {