src: enable writev for pipe handles on Unix

This commit enables writev for Unix Domain Sockets on supported
platforms thus enabling cork/uncork functionality for them and
improving IPC performance.

Fixes: https://github.com/nodejs/node/issues/5095
PR-URL: https://github.com/nodejs/node/pull/10677


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>