std: Remove #[allow] directives in sys modules

These were suppressing lots of interesting warnings! Turns out there was also
quite a bit of dead code.

src/libstd/sys/common/net.rs

@@ -26,7 +26,9 @@ use str;
 use sys::{self, retry, c, sock_t, last_error, last_net_error, last_gai_error, close_sock,
           wrlen, msglen_t, os, wouldblock, set_nonblocking, timer, ms_to_timeval,
           decode_error_detailed};
-use sync::{Arc, Mutex, MutexGuard};
+use sync::{Arc, Mutex};
+#[cfg(not(target_os = "linux"))]
+use sync::MutexGuard;
 use sys_common::{self, keep_going, short_write, timeout};
 use cmp;
 use old_io;
@@ -620,11 +622,13 @@ impl Drop for Inner {
     fn drop(&mut self) { unsafe { close_sock(self.fd); } }
 }
 
+#[cfg(not(target_os = "linux"))]
 pub struct Guard<'a> {
     pub fd: sock_t,
     pub guard: MutexGuard<'a, ()>,
 }
 
+#[cfg(not(target_os = "linux"))]
 #[unsafe_destructor]
 impl<'a> Drop for Guard<'a> {
     fn drop(&mut self) {