Fixes bug #4802: http://bugs.alpinelinux.org/issues/4802

Seems like iscsiadm relies on checking optopt for checking if there's an error, instead of checking whether opterr is set or if getopt returns '?' and then using optopt to retrieve the option that it didn't recognize.

For some reason musl sets optopt to the last option parsed even if it was valid. Is musl supposed to do that? glibc doesn't behave this way. The posix standard for getopt also reads like optopt is only supposed to be set when there's an error.

(Only when I was looking for what to put in the email subject line, after I figured it all out did I notice that Brian Angus already submitted a fix earlier. Figured I'd sent this one anyway; that fix removes error output when presented with actual unrecognised arguments.)