fix coap_socket_strerror() on Windows

[mkicherer]

The Windows version of coap_socket_strerror calls coap_socket_format_errno with the result of WSAGetLastError. At least in my specific case, this results in an "unknown error" message as WSAGetLastError seems to use different numbers than strerror.

coap_io_process_with_fds_lkd tries to translate WSAGetLastError to errno with coap_win_error_to_errno but the Windows version of coap_socket_strerror does not use errno, so coap_win_error_to_errno is called in vain.

With this patch, coap_win_error_to_errno is always called in the Windows version of coap_socket_strerror and then errnois passed to coap_socket_format_errno and therefore strerror.

[mkicherer]

Sorry, coap_win_error_to_errno is obviously needed in coap_io_process_with_fds_lkd. Please ignore this part of the comment.

[mrdeep1]

The update change looks fine to me.

Looks like git.savannah.nongnu.org is currently down for doing the LwIP CI test.