signal: Add SignalKind for registering signals more easily (#1430)

This avoids having consumers import libc for common signals, and it
improves discoverability since users need not be aware that libc
contains all supported constants.

tokio-signal/tests/drop_multi_loop.rs

@@ -6,18 +6,18 @@ use libc;
 pub mod support;
 use crate::support::*;
 
-const TEST_SIGNAL: libc::c_int = libc::SIGUSR1;
-
 #[test]
 fn dropping_loops_does_not_cause_starvation() {
     let (mut rt, signal) = {
+        let kind = SignalKind::sigusr1();
+
         let mut first_rt = CurrentThreadRuntime::new().expect("failed to init first runtime");
-        let mut first_signal = Signal::new(TEST_SIGNAL).expect("failed to register first signal");
+        let mut first_signal = Signal::new(kind).expect("failed to register first signal");
 
         let mut second_rt = CurrentThreadRuntime::new().expect("failed to init second runtime");
-        let mut second_signal = Signal::new(TEST_SIGNAL).expect("failed to register second signal");
+        let mut second_signal = Signal::new(kind).expect("failed to register second signal");
 
-        send_signal(TEST_SIGNAL);
+        send_signal(libc::SIGUSR1);
 
         let _ = run_with_timeout(&mut first_rt, first_signal.next())
             .expect("failed to await first signal");
@@ -31,7 +31,7 @@ fn dropping_loops_does_not_cause_starvation() {
         (second_rt, second_signal)
     };
 
-    send_signal(TEST_SIGNAL);
+    send_signal(libc::SIGUSR1);
 
     let _ = run_with_timeout(&mut rt, signal.into_future());
 }