MAJOR MOD CHANGE
As before, moderators can silently observe any player that is not observing another player. (configurable)
New: Players can invite other players to observe them. Invitations can be accepted or denied. (configurable privileges, defaults to interact)
All the chat-commands are configurable.
Temporary privileges can be given to observers, same set for sneak-observers and invited ones or separate set.
After leaving the spectator mode in Minetest 5.x, the collisionbox isn't the right size and at the right place.
I simply changed the values of the collisionbox to match the vanilla ones.
A better solution would be to get the current size of the collisionbox when the player enter the spectator mode, store them, and use those when leaving that mode.
This would make the mod more compatible with other mods that changes the player's size.