[feature]: infinite duration?

[Beanzilla]

I'd like to make it so if duration is negative or zero then the priv granted is never revoked.

This would be nice as then you could purchase a priv within-game (kind of like a Rank on some Minecraft server), and have it forever.

Looking at the code, it appears that 2 things would need to be considered:

1. Only minetest.after if the duration is greater than 5 (since you do include a 5 second notice)
2. On buying the boost/priv no tracking should be added, as though the player had the priv in the first place (this way it doesn't get removed on logout/logoff/quit)

It looks like most of this can be obtained via within the buy_boost function, and activate_boost.

I'll make a PR of this separately to my HUD code... though actually it might collide as I'd also not want an infinite duration boost to change the HUD.