Add on_authplayer callback and 'last_login' to on_joinplayer (#9574)

Replace on_auth_fail callback with more versatile on_authplayer
    Better clarify account login process in Lua API documentation
    Change initial timestamp for newly registered accounts to -1

builtin/game/chat.lua

@@ -1068,7 +1068,7 @@ core.register_chatcommand("last-login", {
 			param = name
 		end
 		local pauth = core.get_auth_handler().get_auth(param)
-		if pauth and pauth.last_login then
+		if pauth and pauth.last_login and pauth.last_login ~= -1 then
 			-- Time in UTC, ISO 8601 format
 			return true, "Last login time was " ..
 				os.date("!%Y-%m-%dT%H:%M:%SZ", pauth.last_login)