Re-enable networking module fo rL4Re

As suggested in the discussion of PR rust-lang#43972, std should provide a uniform API to
all platforms. Since there's no networking on L4Re, this now is a module in
`sys::net` providing types and functions/methods returning an error for each
action.

Author: humenda

src/libstd/lib.rs

@@ -469,7 +469,6 @@ pub mod error;
 pub mod ffi;
 pub mod fs;
 pub mod io;
-#[cfg(not(target_os = "l4re"))]
 pub mod net;
 pub mod num;
 pub mod os;

src/libstd/sys/unix/ext/mod.rs

@@ -36,7 +36,6 @@ pub mod fs;
 pub mod process;
 pub mod raw;
 pub mod thread;
-#[cfg(not(target_os = "l4re"))]
 pub mod net;
 
 /// A prelude for conveniently writing platform-specific code.