Add support for -h/--help options.

RHBZ#962168. In response to user request added support for invoking with the -h
or --help argument to print usage information. Previously usage was only
printed if the user provided less than two arguments AND did not provide any
invalid arguments, as they were unhandled the commonly used -h and --help
arguments counted as invalid.

https://bugzilla.redhat.com/show_bug.cgi?id=962168

Signed-off-by: Heikki Hokkanen <hoxu@users.sf.net>
Refer to manual page in usage.