added -l --linktype flag

It can be used to restrict the set of interfaces to a particular
linktype.
Merge branch 'linktype' of https://github.com/hgrohne-cygnusnetworks-de/check_iftraffic_nrpe.py into hgrohne-cygnusnetworks-de-linktype

Conflicts:
	check_iftraffic_nrpe.py

I am merging this pull request: https://github.com/samyboy/check_iftraffic_nrpe.py/pull/11