net-firewall/nftables: rename modern_kernel use flag

net-firewall/nftables: rename modern_kernel use flag

When the new scripts supporting atomic updates on modern kernels
where introduced, usage of the new scripts was controlled by the
modern_kernel USE flag.

QA has requested that we avoid using underscores on USE flags as
they are a reserved character.

Rename the modern_kernel USE flag to modern-kernel to address
this issue.

Closes: https://bugs.gentoo.org/694838
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>