net-firewall/nftables: Port missing changes to 0.9.3

net-firewall/nftables: Port missing changes to 0.9.3

Lars added nftables-0.9.3 at the time I was doing the changes
to rename the modern_kernel USE flag and move the skel files
into its own folder in doc.

As a result of this, I missed the ebuild and didn't update it.
Update also the 0.9.3 ebuild so it follows the same conventions
as the others.

Signed-off-by: Francisco Blas Izquierdo Riera (klondike) <klondike@gentoo.org>
Package-Manager: Portage-2.3.76, Repoman-2.3.11
Signed-off-by: Matthew Thode <prometheanfire@gentoo.org>