The fallback for systems that lack preadv() only filled the first
buffer.
This commit rectifies that to fill all (or at least as many as possible)
buffers.
Fixes: https://github.com/libuv/libuv/issues/2332
PR-URL: https://github.com/libuv/libuv/pull/2338
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Jameson Nash <vtjnash@gmail.com>
Reviewed-By: Saúl Ibarra Corretgé <saghul@gmail.com>
