With their joints creaking almost as much as the script, this instalment of the franchise sees “Riggs” (Mel Gibson) and “Murtaugh” (Danny Glover) start they way they mean to go on - pyrotechnically! This time, it’s the encroaching Triad gangs from China who are involved in some people smuggling and currency forging that could end up facilitating an all-out war in Chinatown between the menacingly stylish “Wah Sing…