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.

doc/lua_api.txt

@@ -1998,6 +1998,8 @@ and `minetest.auth_reload` call the authetification handler.
     * `val` is between `0` and `1`; `0` for midnight, `0.5` for midday
 * `minetest.get_timeofday()`
 * `minetest.get_gametime()`: returns the time, in seconds, since the world was created
+* `minetest.get_day_count()`: returns number days elapsed since world was created,
+    * accounting for time changes.
 * `minetest.find_node_near(pos, radius, nodenames)`: returns pos or `nil`
     * `radius`: using a maximum metric
     * `nodenames`: e.g. `{"ignore", "group:tree"}` or `"default:dirt"`