mnexec: getopt bugfix

getopt returns an 'int', so use change datatype of 'c' to match it.
Otherwise, 'c' may hold a value of 255 (0xff), and fail the comparison
with -1 (0xffffffff): while ((c = getopt(...)) != -1)

This bug was uncovered on Ubuntu 13.04 running on ARM, using
arm-linux-gnueabihf-gcc4.7.