crypto: fix style issues in bio

Stop changing arguments, use local variables for things that change.
crypto: fix changing buffers in bio

We should go to next buffer if *current* one is full, not the next one.
Otherwise we may hop through buffers and written data will become
interleaved, which will lead to failure.
crypto: move write_head in bio's Reset() method
streams2: coalesce mode and writev
net: switch socket into coalesce mode, _writev

Introduce os_binding.getIovMax() to limit buffer count when
calling _writev().
http: write buffers when chunked to embrace writev