kernel/postinst.d: check for correct update-initramfs provider

Ubuntu's dracut package ships an update-initramfs command with the same interface as initramfs-tools'. The difference is that it calls dracut instead of mkinitramfs.

When installing dracut without purging initramfs-tools, /etc/kernel/postinst.d/initramfs-tools will still execute update-initramfs (in addition to dracut's kernel postinst hook).

So query the newly add --version parameter to determine which package provided the update-initramfs script. Only run the kernel postinst hook when the provider matches.