fix 2GB file transfer limit

[brettsheffield]

Due to an unsafe cast in wolfSSL_writev() the total amount of data that can be sent at once is INT_MAX bytes despite the API allowing an array of up to (INT_MAX * size_t) chunks to be passed in.

The result is files greater than INT_MAX bytes (~2GB) cannot be sent (including headers).

Work around this limitation, and consider sending a patch to wolfssl to fix this brokeness.