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/sys.rs

@@ -0,0 +1,35 @@
+pub use libc::{c_void, socklen_t, SYSPROTO_CONTROL, IFNAMSIZ, sockaddr_ctl, AF_SYSTEM, AF_SYS_CONTROL};
+use nix::ioctl_readwrite;
+
+pub const UTUN_CONTROL_NAME: &str = "com.apple.net.utun_control";
+pub const UTUN_OPT_IFNAME: libc::c_int = 2;
+
+pub const MAX_KCTL_NAME: usize = 96;
+
+    #[repr(C)]
+pub struct ctl_info {
+    pub ctl_id: u32,
+    pub ctl_name: [u8; MAX_KCTL_NAME],
+}
+
+#[repr(C)]
+pub struct ifreq {}
+
+ioctl_readwrite!(resolve_ctl_info, b'N', 3, ctl_info);
+
+/// Copied from https://github.com/rust-lang/socket2/blob/61314a231f73964b3db969ef72c0e9479df320f3/src/sys/unix.rs#L168-L178
+/// getsockopt is not exposed by socket2
+#[macro_export]
+macro_rules! syscall {
+    ($fn: ident ( $($arg: expr),* $(,)* ) ) => {{
+        #[allow(unused_unsafe)]
+        let res = unsafe { libc::$fn($($arg, )*) };
+        if res == -1 {
+            Err(std::io::Error::last_os_error())
+        } else {
+            Ok(res)
+        }
+    }};
+}
+
+pub use syscall;