std: Set CLOEXEC for all fds opened on unix

This commit starts to set the CLOEXEC flag for all files and sockets opened by
the standard library by default on all unix platforms. There are a few points of
note in this commit:

* The implementation is not 100% satisfactory in the face of threads. File
  descriptors only have the `F_CLOEXEC` flag set *after* they are opened,
  allowing for a fork/exec to happen in the middle and leak the descriptor.
  Some platforms do support atomically opening a descriptor while setting the
  `CLOEXEC` flag, and it is left as a future extension to bind these apis as it
  is unclear how to do so nicely at this time.

* The implementation does not offer a method of opting into the old behavior of
  not setting `CLOEXEC`. This will possibly be added in the future through
  extensions on `OpenOptions`, for example.

* This change does not yet audit any Windows APIs to see if the handles are
  inherited by default by accident.

This is a breaking change for users who call `fork` or `exec` outside of the
standard library itself and expect file descriptors to be inherted. All file
descriptors created by the standard library will no longer be inherited.

[breaking-change]

src/libstd/sys/unix/net.rs

@@ -47,7 +47,9 @@ impl Socket {
         };
         unsafe {
             let fd = try!(cvt(libc::socket(fam, ty, 0)));
-            Ok(Socket(FileDesc::new(fd)))
+            let fd = FileDesc::new(fd);
+            fd.set_cloexec();
+            Ok(Socket(fd))
         }
     }
 
@@ -56,13 +58,16 @@ impl Socket {
         let fd = try!(cvt_r(|| unsafe {
             libc::accept(self.0.raw(), storage, len)
         }));
-        Ok(Socket(FileDesc::new(fd)))
+        let fd = FileDesc::new(fd);
+        fd.set_cloexec();
+        Ok(Socket(fd))
     }
 
     pub fn duplicate(&self) -> io::Result<Socket> {
-        cvt(unsafe { libc::dup(self.0.raw()) }).map(|fd| {
-            Socket(FileDesc::new(fd))
-        })
+        let fd = try!(cvt(unsafe { libc::dup(self.0.raw()) }));
+        let fd = FileDesc::new(fd);
+        fd.set_cloexec();
+        Ok(Socket(fd))
     }
 
     pub fn read(&self, buf: &mut [u8]) -> io::Result<usize> {