Switch example NSD config to splitting master and slave zones into different

Switch example NSD config to splitting master and slave zones into different
subdirectories (/var/nsd/zones/{master,slave}) and create these in mtree.
Nearly everybody that uses NSD for slave zones that I talked to already has
this layout. Bikesh^Wdiscussed with ajacoutot florian millert and others.

ok ajacoutot@ florian@ phessler@ claudio@ jung@