(#15586) Use OptionParser for help

Ruby 1.9.3 dropped Rdoc.usage. The output of `facter --help` depended on
this command. The suggested change from the Rdoc maintainer is to use
the banner of the OptionParser instead. This changes it to use the
banner.
Merge pull request #522 from zaphod42/issue/stable/help-on-1.9.3

(#15586) Use OptionParser for help