Implement fmt::Debug for all structures in libstd.

Part of https://github.com/rust-lang/rust/issues/31869.

Also turn on the `missing_debug_implementations` lint at the crate
level.

src/libstd/os/linux/raw.rs

@@ -17,6 +17,7 @@
                               crates.io should be used instead for the correct \
                               definitions")]
 #![allow(deprecated)]
+#![allow(missing_debug_implementations)]
 
 use os::raw::c_ulong;