std: Expose that LocalIo may not always be available

It is not the case that all programs will always be able to acquire an instance
of the LocalIo borrow, so this commit exposes this limitation by returning
Option<LocalIo> from LocalIo::borrow().

At the same time, a helper method LocalIo::maybe_raise() has been added in order
to encapsulate the functionality of raising on io_error if there is on local I/O
available.

src/libstd/io/net/unix.rs

@@ -59,14 +59,9 @@ impl UnixStream {
     ///     stream.write([1, 2, 3]);
     ///
     pub fn connect<P: ToCStr>(path: &P) -> Option<UnixStream> {
-        let mut io = LocalIo::borrow();
-        match io.get().unix_connect(&path.to_c_str()) {
-            Ok(s) => Some(UnixStream::new(s)),
-            Err(ioerr) => {
-                io_error::cond.raise(ioerr);
-                None
-            }
-        }
+        LocalIo::maybe_raise(|io| {
+            io.unix_connect(&path.to_c_str()).map(UnixStream::new)
+        })
     }
 }
 
@@ -107,14 +102,9 @@ impl UnixListener {
     ///     }
     ///
     pub fn bind<P: ToCStr>(path: &P) -> Option<UnixListener> {
-        let mut io = LocalIo::borrow();
-        match io.get().unix_bind(&path.to_c_str()) {
-            Ok(s) => Some(UnixListener{ obj: s }),
-            Err(ioerr) => {
-                io_error::cond.raise(ioerr);
-                None
-            }
-        }
+        LocalIo::maybe_raise(|io| {
+            io.unix_bind(&path.to_c_str()).map(|s| UnixListener { obj: s })
+        })
     }
 }