change getopt return from char to int

according to ```man 3 getopt``` getopt returns an int which gets -1 when no more options are available. You used to store the return value of getopt in a char, which is a unsigned 8 bit integer. As such it can actually never get -1.

So on my RasPi ```gcc version 4.6.3 (Debian 4.6.3-14+rpi1)``` did not leave the while loop correctly, even when no option was specified.