net-firewall/nftables: add doc and xtables USE flags

net-firewall/nftables: add doc and xtables USE flags

This commit adds some fixes on the 0.9.1 package:
* Adds the doc USE flag to control building of the man pages
* Adds the app-text/asciidoc when building the man pages
* Adds the xtables USE flag for better compatibility with iptables-compat
* Adds the net-firewall/iptables dependency when using the xtables flag

I have chosen +doc, xtables for better compatibility with prior versions.
Also whilst man pages generally make sense, the xtables USE isn't needed
on pure nftables setups which are the recommended approach.

Closes: https://bugs.gentoo.org/688952
Signed-off-by: Francisco Blas Izquierdo Riera (klondike) <klondike@gentoo.org>
Package-Manager: Portage-2.3.66, Repoman-2.3.11
Signed-off-by: Matthew Thode <prometheanfire@gentoo.org>