When a WG connection is connecting to an IPv6 endpoint, configures a
default route, and firewalld is active with IPv6_rpfilter=yes, it never
handshakes and doesn't pass traffic. This is because firewalld has a
IPv6 reverse path filter which is discarding these packets.
Thus, we add some firewall rules whenever a WG connection is brought up
that ensure the conntrack mark and packet mark are copied over.
These rules are largely inspired by wg-quick:
https://git.zx2c4.com/wireguard-tools/tree/src/wg-quick/linux.bash?id=17c78d31c27a3c311a2ff42a881057753c6ef2a4#n221
(cherry picked from commit db557908a2)
Sync/blocking methods are ugly. Their name should highlight this.
Also, we may have an async variant, so we will need the "good" name
for apply() and apply_finish().
It's still not a very good name, but it seems better then
NMUtilsShareRules.
Currently, NMFirewallConfig is mostly about masquerading for shared
mode. But in practice, it's a piece of configuration for something to
configure in the firewall (the NAT and filter rules).
