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...)

lvm.c

@@ -1565,7 +1565,7 @@ void luaV_execute (lua_State *L, CallInfo *ci) {
           if (nparams1)  /* vararg function? */
             delta = ci->u.l.nextraargs + nparams1;
           /* close upvalues from current call */
-          luaF_close(L, base, -1);  /* (no to-be-closed vars. here) */
+          luaF_close(L, base, LUA_OK);
           updatestack(ci);
         }
         if (!ttisfunction(s2v(ra))) {  /* not a function? */