Add consistent monotonic day counter - get_day_count()

I've written several experimental bits of code that revolve around the
need for a consistent calendar, but implementing one is extremely hard
in mods due to time changes and mods overriding core.get_timeofday(),
which will conflict.

The second part of the problem is that doing this from a mod requires
constant maintenance of a settings file.

An implementation in core is trivial, however, and solves all of
these problems at virtually no cost: No extra branches in server
steps, and a single branch when minetest.set_time_of_day(), which is
entirely reasonable.

We store the day_count value in env_meta.txt.

The use case is obvious: This change allows mods to create an actual
virtual calendar, or properly account for seasonal changes, etc..

We add a "/days" chatcommand that displays the current day count. No
permissions are needed. It can only retrieve the day count, not
modify it.

builtin/game/chatcommands.lua

@@ -785,6 +785,13 @@ core.register_chatcommand("time", {
 	end,
 })
 
+core.register_chatcommand("days", {
+	description = "Display day count",
+	func = function(name, param)
+		return true, "Current day is " .. core.get_day_count()
+	end
+})
+
 core.register_chatcommand("shutdown", {
 	description = "shutdown server",
 	privs = {server=true},