examples: ignore interrupted syscalls for mio poll

While in general these examples shouldn't be written to handle errors,
the long-running MIO poll operation is especially prone to returning
interrupted syscall errors when a debugger is attached.

This commit updates each MIO example to ignore this class of error
rather than panicing, improving the debugging experience.

examples/src/bin/tlsclient-mio.rs

@@ -534,7 +534,14 @@ fn main() {
     tlsclient.register(poll.registry());
 
     loop {
-        poll.poll(&mut events, None).unwrap();
+        match poll.poll(&mut events, None) {
+            Ok(_) => {}
+            // Polling can be interrupted (e.g. by a debugger) - retry if so.
+            Err(e) if e.kind() == io::ErrorKind::Interrupted => continue,
+            Err(e) => {
+                panic!("poll failed: {:?}", e)
+            }
+        }
 
         for ev in events.iter() {
             tlsclient.ready(ev);

examples/src/bin/tlsserver-mio.rs

@@ -702,7 +702,14 @@ fn main() {
 
     let mut events = mio::Events::with_capacity(256);
     loop {
-        poll.poll(&mut events, None).unwrap();
+        match poll.poll(&mut events, None) {
+            Ok(_) => {}
+            // Polling can be interrupted (e.g. by a debugger) - retry if so.
+            Err(e) if e.kind() == io::ErrorKind::Interrupted => continue,
+            Err(e) => {
+                panic!("poll failed: {:?}", e)
+            }
+        }
 
         for event in events.iter() {
             match event.token() {