Feature Request: Ability to set join-location to a default behavior on child flags.

[ryantheleach]

E.g. I have a town with join-location set, and plots are children of the town.

When a player logs out inside their plot, I want join-location to take no effect.

Currently I can either override it with a value, or let it inherit.

I want to be able to set a 'null' value to this flag, that specifically inherits the flags value as not in effect.