Use symbolic name instead of hardcoded value on telnet(1)

[Ricardo Mestre]

Use symbolic name instead of hardcoded value on telnet(1):

Index: commands.c
===================================================================
RCS file: /cvs/src/usr.bin/telnet/commands.c,v
retrieving revision 1.79
diff -u -p -u -r1.79 commands.c
--- commands.c  13 Nov 2015 17:04:48 -0000      1.79
+++ commands.c  20 Nov 2015 12:30:50 -0000
@@ -1192,7 +1192,7 @@ static void
 close_connection(void)
 {
        if (connected) {
-               (void) shutdown(net, 2);
+               (void) shutdown(net, SHUT_RDWR);
                printf("Connection closed.\r\n");
                (void)close(net);
                connected = 0;