UNIX03 conformance requires utilities to flush stdout before exiting and raise
an error if writing fails. Flushing already happens on a call to exit
and thus automatically on a return from main. Write failure is then
detected by LLVM's default SIGPIPE handler. The handler already exits with
a non-zero code, but conformance additionally requires an error message.
First reapply attempt I hadn't noticed the test had changed, hopefully this
goes better.
Tim Northover