Keep writes within IOV_MAX

[talex5]

Calling Flow.write with more than IOV_MAX buffers results in EINVAL. This can happen when e.g. using Buf_write to write a lot of data without flushing.

Reported by @Cjen1 (see https://github.com/Cjen1/prototyping-testing/tree/master/eio-pipe).

Implementations of Flow.write with such limits should just truncate the list of buffers. Possibly Buf_write should also know the limit and flush before sending so many (or just use a low enough limit that it work on all platforms).

On Linux, ocaml-multicore/ocaml-uring#76 can be used to get the limit.

[smondet]

I think this is the same issue: mirage/ocaml-cohttp#1005

[talex5]

Closing as the original issue is now fixed. I've opened a separate issue for back-pressure: #760