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.

src/script/lua_api/l_env.cpp

@@ -561,6 +561,15 @@ int ModApiEnvMod::l_get_timeofday(lua_State *L)
 	return 1;
 }
 
+// get_day_count() -> int
+int ModApiEnvMod::l_get_day_count(lua_State *L)
+{
+	GET_ENV_PTR;
+
+	lua_pushnumber(L, env->getDayCount());
+	return 1;
+}
+
 // get_gametime()
 int ModApiEnvMod::l_get_gametime(lua_State *L)
 {
@@ -1055,6 +1064,7 @@ void ModApiEnvMod::Initialize(lua_State *L, int top)
 	API_FCT(set_timeofday);
 	API_FCT(get_timeofday);
 	API_FCT(get_gametime);
+	API_FCT(get_day_count);
 	API_FCT(find_node_near);
 	API_FCT(find_nodes_in_area);
 	API_FCT(find_nodes_in_area_under_air);