Mention plink's new -N option in usage and man page.

[originally from svn r4643]

unix/uxplink.c

@@ -230,6 +230,7 @@ static void usage(void)
     printf("  -C        enable compression\n");
     printf("  -i key    private key file for authentication\n");
     printf("  -s        remote command is an SSH subsystem (SSH-2 only)\n");
+    printf("  -N        don't start a shell/command (SSH-2 only)\n");
     exit(1);
 }