This allows writing and reading any amount of buffers,
regardless of what IOV_MAX may be defined as.
It also moves the IOV_MAX test from stream to core.
This is based on the excellent work of @bwijen in #269.
Refs: https://github.com/libuv/libuv/pull/269
PR-URL: https://github.com/libuv/libuv/pull/448
Reviewed-By: Saúl Ibarra Corretgé <saghul@gmail.com>