Add a separate client binary, with some common code to the server.

The cargo workspace thing was a PITA to figure out, but it seems to be working.  Could maybe be simplified?

client/src/main.rs

@@ -0,0 +1,23 @@
+mod options;
+
+use crate::options::OPTIONS;
+use common::{read_string, Request};
+use std::os::unix::net::UnixStream;
+
+fn main() -> std::io::Result<()> {
+    let mut stream = UnixStream::connect(&OPTIONS.socket_name)?;
+    println!("Connected to server");
+
+    let request = Request {
+        userid: OPTIONS.username.clone(),
+        password: OPTIONS.password.clone(),
+        service: "ignore-this".to_string(),
+        realm: "also-ignore-this".to_string(),
+    };
+
+    request.write_to_stream(&mut stream)?;
+    let response = read_string(&mut stream)?;
+    println!("Response: {}", response);
+
+    Ok(())
+}