Send server setting defaultness to client
In 2.6, the server introduced a notion of whether a setting was tracking the server/ruleset default or had an explicitly set value (enum setting_default_level).
The client's server settings storage also has such a notion, but it simply compares the current value to the server's current default at the time the client's server settings are saved (desired_settable_options_update()). This has the risk that if settings are saved when playing a ruleset which happens to have the same default as the user's desired default, the user's preference is lost for other rulesets.
It would be better if the client tracked an explicit notion of defaultness, and the 'Reset' buttons in clients' UI sent '/default' commands, etc.
I think the first step needed for this (before d3f) is to expose (struct setting).setdef over the network.