try to use writev(2) in a non-blocking mode, especially on sockets. Not only
this makes handling of EAGAIN rather weird, but in the case of sockets makes
your code subject of a ENOBUFS, which is absolutely unclear how to handle
properly. *sigh*
Bump PORTREVISION.
Instead of replacing writev(2) with loop based on write(2), test system
limit on the number of vectors writev(2) accepts and split call to writev(2)
to several calls in such a way that number of vectors in each call doesn't
exceed this limit (aka UIO_MAXIOV). Bump PORTREVISION.