Improve options and error handling.

Improve options handling and error out if multiple mutually exclusive
options are passed to acpiconf. Switch from using atoi() to strtol() for
argument parsing, and add error checking and handling, instead of blindly
trusting that the integer conversion is OK.
Cange err() to errx() in once case, the errno value was garbage there.

Reviewed by: emaste
Approved by: emaste
Differential Revision: D13430