Prevent safe construction of struct sigevent

sigevent's Debug, PartialEq, and Hash trait impls might read union
fields that could be potentially uninitialized by a standard
initializer.  Those trait impls shouldn't be present (see
rust-lang#2816), but can't easily be
removed.  Until they get removed, the constructor must be `unsafe` to
force the user to zero all fields.

The same issue applies to the Deref<Target=sigevent_0_2_126> trait impl,
which exists only for backwards compatibility.

src/unix/bsd/freebsdlike/freebsd/mod.rs

@@ -1410,6 +1410,12 @@ s_no_extra_traits! {
         pub sigev_signo: ::c_int,
         pub sigev_value: ::sigval,
         pub _sigev_un: __c_anonymous_sigev_un,
+        /// Exists just to prevent the struct from being safely constructed,
+        /// because the Debug, Hash, PartialImpl, and
+        /// Deref<Target=sigevent_0_2_0126> trait impls might read uninitialized
+        /// fields of _sigev_un.  This field may be removed once those trait
+        /// impls are.
+        _private: ()
     }
 
     pub struct ptsstat {

src/unix/linux_like/mod.rs

@@ -291,6 +291,12 @@ s_no_extra_traits! {
         pub sigev_signo: ::c_int,
         pub sigev_notify: ::c_int,
         pub _sigev_un: __c_anonymous_sigev_un,
+        /// Exists just to prevent the struct from being safely constructed,
+        /// because the Debug, Hash, PartialImpl, and
+        /// Deref<Target=sigevent_0_2_0126> trait impls might read uninitialized
+        /// fields of _sigev_un.  This field may be removed once those trait
+        /// impls are.
+        _private: ()
     }
 }