SECURITY: Fix mediawiki merging accounts

Mediawiki turns any number of _ into whitespace, so Foo__Bar and
Foo_Bar and Foo_Bar_ would be mapped to the same account, leading to
account takeover.

init.lua

@@ -14,3 +14,13 @@ if http then
 	dofile(MP .. "/webmail.lua")
 	mail.webmail_init(http, webmail_url, webmail_key)
 end
+
+
+minetest.register_on_prejoinplayer(function(name, ip)
+      if minetest.player_exists(name) then
+			return
+      end
+      if name:match("_") then
+			return "For security reasons, underscores are banned on this server. Please choose another username!"
+		end
+end)