Begin implementation work on RPC connect points

This is the first stage of #1528 (closed). It provides a backend implementation for RPC connect points in sufficient detail to:

• Handle filesystem operations
• Parse connect points
• Initialize cookie authentication as needed
• Open or bind sockets as required.

There will probably be some conflict with !2615 (merged); I'll fix that once it lands.

It does not handle all of #1528 (closed); see that ticket for next steps.

crates/tor-config-path/src/addr.rs

@@ -67,6 +67,16 @@ impl CfgAddr {
         }
     }
 
+    /// Return true if this address is of a type to which variable substitutions will apply.
+    ///
+    /// Currently, substitutions apply to Unix addresses but not to Inet addresses.
+    pub fn substitutions_will_apply(&self) -> bool {
+        match &self.0 {
+            AddrInner::Inet(_) => false,
+            AddrInner::Unix(_) => true,
+        }
+    }
+
     /// Helper: if possible, format this address as a String.
     ///
     /// (This will return Err(p) if this path is a literal unix path