implement Default for UnixCredentials on Linux

Author: asomers

CHANGELOG.md

@@ -35,7 +35,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
   (#[1252](https://github.com/nix-rust/nix/pull/1252))
 - Added support for `Ipv4PacketInfo` and `Ipv6PacketInfo` to `ControlMessage`.
   (#[1222](https://github.com/nix-rust/nix/pull/1222))
-- `CpuSet` now implements `Default`.
+- `CpuSet` and `UnixCredentials` now implement `Default`.
   (#[1244](https://github.com/nix-rust/nix/pull/1244))
 
 ### Changed

src/sys/socket/mod.rs

@@ -221,6 +221,12 @@ cfg_if! {
             }
         }
 
+        impl Default for UnixCredentials {
+            fn default() -> Self {
+                Self::new()
+            }
+        }
+
         impl From<libc::ucred> for UnixCredentials {
             fn from(cred: libc::ucred) -> Self {
                 UnixCredentials(cred)