an addition to options.h

[NathanTech7713]

Could we have an addition to options.h to say whether to use the last_move prop or not? It's useful on smaller databases, but can really blote bigger ones, :) It's a great feature though. Would be even better if we could have it in $server_options.enable_last_move, but I'm thinking that could get complicated.

[distantorigin]

I added a startup flag ('-m') in a8218d9 to clear all existing last_move properties, and never set them for the duration of the server's lifetime.