Fix the tests in FreeBSD jails (#815)
In a shared-IP FreeBSD jail, "127.0.0.1" is just an alias for the jail's
IP, causing the net::udp::UdpSocket::local_addr test to fail. Disable
that test on FreeBSD.
diff --git a/src/net/udp.rs b/src/net/udp.rs
index e5d90a2..c0fd6d1 100644
--- a/src/net/udp.rs
+++ b/src/net/udp.rs
@@ -145,6 +145,11 @@
///
/// # Examples
///
+ // This assertion is almost, but not quite, universal. It fails on
+ // shared-IP FreeBSD jails. It's hard for mio to know whether we're jailed,
+ // so simply disable the test on FreeBSD.
+ #[cfg_attr(not(target_os = "freebsd"), doc = " ```")]
+ #[cfg_attr(target_os = "freebsd", doc = " ```no_run")]
/// ```
/// # use std::error::Error;
/// #
