libwebsockets

esp32: align with esp-idf changes

LWIP_SOCKET_OFFSET is now nonzero, which I handled a while back.

But the C api support for it is broken in esp-idf.

select() takes unmodified fds, but FD_SET / FD_ISSET etc must have the
offset subtracted on their args.

With this we are working on current HEAD esp-idf.