check_http: Document the new -E option

Mention the -E/--extended-perfdata flag in the --help output.
Bug fix for ncpus if hyperthreading turned off in linux
Merge branch 'master' of https://github.com/bbense/nagios-plugins

* 'master' of https://github.com/bbense/nagios-plugins:
  Bug fix for ncpus if hyperthreading turned off in linux
Clean up check for _SC_NPROCESSORS_ONLN

Closes #62.