Haiku: add workaround for missing F_DUPFD_CLOEXEC

The src/libstd/sys/unix/net.rs file depends on it. It is missing
from Haiku. This workaround should actually 'fix' the problem,
but it turns out the fds-are-cloexec.rs test hangs. I do not
know how related these two issues are, but it warrants further
investigation.

The test is ignored on this platform for now.

* Hand rebased from Niels original work on 1.9.0

Author: nielx

src/test/run-pass/fds-are-cloexec.rs

@@ -11,6 +11,7 @@
 // ignore-windows
 // ignore-android
 // ignore-emscripten
+// ignore-haiku
 
 #![feature(libc)]