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/mod.rs

@@ -0,0 +1,29 @@
+mod queue;
+
+#[cfg(target_vendor = "apple")]
+#[path = "apple/mod.rs"]
+mod imp;
+
+#[cfg(target_os = "linux")]
+#[path = "linux/mod.rs"]
+mod imp;
+
+pub use imp::TunInterface;
+pub use queue::TunQueue;
+
+pub fn ifname_to_string(buf: [libc::c_char; libc::IFNAMSIZ]) -> String {
+    // TODO: Switch to `CStr::from_bytes_until_nul` when stabilized
+    unsafe {
+        std::ffi::CStr::from_ptr(buf.as_ptr() as *const _)
+            .to_str()
+            .unwrap()
+            .to_string()
+    }
+}
+
+pub fn string_to_ifname(name: &str) -> [libc::c_char; libc::IFNAMSIZ] {
+    let mut buf = [0 as libc::c_char; libc::IFNAMSIZ];
+    let len = name.len().min(buf.len());
+    buf[..len].copy_from_slice(unsafe { &*(name.as_bytes() as *const _ as *const [libc::c_char]) });
+    buf
+}