Implement IPv4 address configuration on Linux

This involved refactoring the crate structure to share code between
macOS and Linux. The new methods have not yet been implemented on
macOS, but they have todo!() placeholders.

tun/Cargo.toml

@@ -5,6 +5,7 @@ edition = "2021"
 
 [dependencies]
 libc = "0.2"
+fehler = "1.0"
 nix = { version = "0.25", features = ["ioctl"] }
 socket2 = "0.4"
 tokio = { version = "1.21", features = [] }