socket set_mark addition.

to be able to set a marker/id on the socket for network filtering
 (iptables/ipfw here) purpose.

library/std/src/os/unix/net/stream.rs

@@ -424,6 +424,12 @@ impl UnixStream {
         self.0.passcred()
     }
 
+    #[cfg(any(doc, target_os = "linux", target_os = "freebsd",))]
+    #[unstable(feature = "unix_set_mark", issue = "none")]
+    pub fn set_mark(&self, mark: u32) -> io::Result<()> {
+        self.0.set_mark(mark)
+    }
+
     /// Returns the value of the `SO_ERROR` option.
     ///
     /// # Examples

library/std/src/sys/unix/net.rs

@@ -427,6 +427,16 @@ impl Socket {
         self.0.set_nonblocking(nonblocking)
     }
 
+    #[cfg(target_os = "linux")]
+    pub fn set_mark(&self, mark: u32) -> io::Result<()> {
+        setsockopt(self, libc::SOL_SOCKET, libc::SO_MARK, mark as libc::c_int)
+    }
+
+    #[cfg(target_os = "freebsd")]
+    pub fn set_mark(&self, mark: u32) -> io::Result<()> {
+        setsockopt(self, libc::SOL_SOCKET, libc::SO_USER_COOKIE, mark)
+    }
+
     pub fn take_error(&self) -> io::Result<Option<io::Error>> {
         let raw: c_int = getsockopt(self, libc::SOL_SOCKET, libc::SO_ERROR)?;
         if raw == 0 { Ok(None) } else { Ok(Some(io::Error::from_raw_os_error(raw as i32))) }