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?

server/src/server.rs

@@ -0,0 +1,34 @@
+use crate::handler::FileHandler;
+use common::{Request, RESPONSE_NO, RESPONSE_OK};
+use std::io::{Error, Write};
+use std::os::unix::net::UnixStream;
+
+pub struct Server {
+    stream: UnixStream,
+    handler: FileHandler,
+}
+
+impl Server {
+    pub fn new(stream: UnixStream, handler: FileHandler) -> Result<Server, Error> {
+        Ok(Server { stream, handler })
+    }
+
+    fn communicate(&mut self) -> Result<(), Error> {
+        let request = Request::from_stream(&mut self.stream)?;
+
+        println!("userid: {}", request.userid);
+        if self.handler.check_auth(&request) {
+            self.stream.write_all(&RESPONSE_OK)?
+        } else {
+            self.stream.write_all(&RESPONSE_NO)?
+        }
+
+        Ok(())
+    }
+
+    pub fn handle_client(&mut self) {
+        if let Err(error) = self.communicate() {
+            eprint!("{error}");
+        }
+    }
+}