This fixes an issue introduced by https://reviews.freebsd.org/D33246

getopt returns an int, which was stored inside c of type char. c was then compared to -1. All those type conversions worked fine on our amd64 systems, but not on armv6 architecture.

The -1 returned by getopt, stored in c, never successfully compared to -1, causing the program to fall into the usage error in the default: switch case.

I also fixed the usage function, as -C takes a path as parameter.