Add the world seed to the startup logging (#4295)

In multiplayer, only the server logs this information - the client has no access to it.

Reviewed-on: VoxeLibre/VoxeLibre#4295
Reviewed-by: the-real-herowl <the-real-herowl@noreply.git.minetest.land>
Co-authored-by: Nicu <kneekoo@noreply.git.minetest.land>
Co-committed-by: Nicu <kneekoo@noreply.git.minetest.land>

mods/CORE/mcl_init/init.lua

@@ -1,6 +1,8 @@
 -- Some global variables (don't overwrite them!)
 mcl_vars = {}
 
+minetest.log("action", "World seed = " .. minetest.get_mapgen_setting("seed"))
+
 mcl_vars.redstone_tick = 0.1
 
 -- GUI / inventory menu settings