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/auth.lua

@@ -41,7 +41,6 @@ core.builtin_auth_handler = {
 		return {
 			password = auth_entry.password,
 			privileges = privileges,
-			-- Is set to nil if unknown
 			last_login = auth_entry.last_login,
 		}
 	end,
@@ -53,7 +52,7 @@ core.builtin_auth_handler = {
 			name = name,
 			password = password,
 			privileges = core.string_to_privs(core.settings:get("default_privs")),
-			last_login = os.time(),
+			last_login = -1,  -- Defer login time calculation until record_login (called by on_joinplayer)
 		})
 	end,
 	delete_auth = function(name)