New functions 'lua_resetthread' and 'coroutine.kill'

New functions to reset/kill a thread/coroutine, mainly (only?) to
close any pending to-be-closed variable. ('lua_resetthread' also
allows a thread to be reused...)

ltests.c

@@ -1366,6 +1366,9 @@ static int runC (lua_State *L, lua_State *L1, const char *pc) {
     else if EQ("newthread") {
       lua_newthread(L1);
     }
+    else if EQ("resetthread") {
+      lua_pushinteger(L1, lua_resetthread(L1));
+    }
     else if EQ("newuserdata") {
       lua_newuserdata(L1, getnum);
     }