Ctrl+C handling (#75)

As stated [here](https://doc.rust-lang.org/std/io/trait.BufRead.html#errors-1), `read_until` (and `read_line`) ignore Ctrl+C, so Topgrade does not respond to Ctrl+C in the retry prompt, and instead will exit only when enter is pressed after Ctrl+C. This is undesirable, so this pull request is a WIP until we find a solution.

src/ctrlc/unix.rs

@@ -0,0 +1,29 @@
+use nix::sys::signal;
+use std::sync::atomic::{AtomicBool, Ordering};
+
+lazy_static! {
+    static ref RUNNING: AtomicBool = AtomicBool::new(true);
+}
+
+pub fn running() -> bool {
+    RUNNING.load(Ordering::SeqCst)
+}
+
+pub fn set_running(value: bool) {
+    RUNNING.store(value, Ordering::SeqCst)
+}
+
+extern "C" fn handle_sigint(_: i32) {
+    set_running(false);
+}
+
+pub fn set_handler() {
+    let sig_action = signal::SigAction::new(
+        signal::SigHandler::Handler(handle_sigint),
+        signal::SaFlags::empty(),
+        signal::SigSet::empty(),
+    );
+    unsafe {
+        signal::sigaction(signal::SIGINT, &sig_action).unwrap();
+    }
+}