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).
