sys::modules => use directly kmod on Debian 9 and beyond

On Debian 9 and beyond the original module-init-tools is not available anymore: on Jessie is still marked as a transitional package but on Stretch,etc. it has to be substituted by kmod.

I just want to use the sys::module recipe on the HVS hosts installed with Stretch, in order to load an additional module requested by S.Fleisch: Sharing Host files with the Guest.

NOTE: despite the name Plan 9 folder sharing over Virtio, the Plan 9 OS is not involved.