Fixed SF.net bug 3560976, added --include-type/-N option

Hi nagios-plugins Team,
here's the PullRequest fixing SourceForge.net bug #3560976.
It adds an --include-type option to check_disk.

Here's the original text of the bugreport:
> Hi.
> 
> It would be great if, in addition to -X, --exclude-type=TYPE, there was a --include-type option....
> If include-type is given (first) then per default no fs type shall be included but the ones given there.
> 
> Cheers,
> Chris.

regards,
Richard