Update ch3_fork a small bug, and add pthread sample code

Guo Wenxue

2018-11-15 f216a409197724504799ee59720f9ee19e84060c

 ch3_fork/socket_server.c

@@ -142,7 +142,7 @@
         rv=write(clifd, MSG_STR, strlen(MSG_STR));
                if(rv < 0)
                {
                   printf("Write to client by sockfd[%d] failure: %s\n", sockfd, strerror(errno));
                   printf("Write to client by sockfd[%d] failure: %s\n", clifd, strerror(errno));
                   close(clifd);
            exit(0);
                }