hosts/gentoo: recognize systemd for nfsd startup and replace "service" command with /etc/init.d/nfs

Gentoo does not use the "service" command for init scripts rather script execution is done through /etc/init.d/. That will still not work for users using systemd. The patch checks for the presence of systemd from the current processes and starts the services. The service units used in the patch are available through official Gentoo nfs packages.