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/src/unix/apple/mod.rs

@@ -0,0 +1,76 @@
+use fehler::throws;
+use libc::c_char;
+use std::net::Ipv4Addr;
+use std::os::unix::io::{AsRawFd, IntoRawFd, RawFd};
+use std::io::Error;
+
+mod sys;
+mod kern_control;
+
+pub use super::queue::TunQueue;
+
+use super::ifname_to_string;
+use kern_control::SysControlSocket;
+
+#[derive(Debug)]
+pub struct TunInterface {
+    pub(crate) socket: socket2::Socket,
+}
+
+impl TunInterface {
+    #[throws]
+    pub fn new() -> TunInterface {
+        TunInterface::connect(0)?
+    }
+
+    #[throws]
+    fn connect(index: u32) -> TunInterface {
+        use socket2::{Domain, Protocol, Socket, Type};
+
+        let socket = Socket::new(
+            Domain::from(sys::AF_SYSTEM),
+            Type::DGRAM,
+            Some(Protocol::from(sys::SYSPROTO_CONTROL)),
+        )?;
+        let addr = socket.resolve(sys::UTUN_CONTROL_NAME, index)?;
+        socket.connect(&addr)?;
+
+        TunInterface { socket }
+    }
+
+    #[throws]
+    pub fn name(&self) -> String {
+        let mut buf = [0 as c_char; sys::IFNAMSIZ];
+        let mut len = buf.len() as sys::socklen_t;
+        sys::syscall!(getsockopt(
+            self.as_raw_fd(),
+            sys::SYSPROTO_CONTROL,
+            sys::UTUN_OPT_IFNAME,
+            buf.as_mut_ptr() as *mut sys::c_void,
+            &mut len,
+        ))?;
+        ifname_to_string(buf)
+    }
+
+    #[throws]
+    pub fn set_ipv4_addr(&self, _addr: Ipv4Addr) {
+        todo!()
+    }
+
+    #[throws]
+    pub fn ipv4_addr(&self) -> Ipv4Addr {
+        todo!()
+    }
+}
+
+impl AsRawFd for TunInterface {
+    fn as_raw_fd(&self) -> RawFd {
+        self.socket.as_raw_fd()
+    }
+}
+
+impl IntoRawFd for TunInterface {
+    fn into_raw_fd(self) -> RawFd {
+        self.socket.into_raw_fd()
+    }
+}