Update mlvpn.c to make it work for uClibc1.014_eabi of my armv7 CPU

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c = getopt_long(argc, argv, optstr, long_options, &option_index); when c is char type it will return -1 in x86 and mips CPU but it will return 255 all the time and I never get -1 in my armv7 CPU with uClibc1.014_eabi . So the program will blocked in preset_permitted() and never go next. But when I correct the c as int type. It works fine for me.