init: update listeners when pressing <ENTER> on the serial console

This is useful when the network doesn’t come up correctly: you can connect a
network cable between your computer and the gokrazy machine, press <ENTER> on
the serial console and connect to gokrazy via its IPv6 link-local address (to
find the address, use ping6 ff02::1%eth0).

gokrazy.go

@@ -8,6 +8,7 @@
 package gokrazy
 
 import (
+	"bufio"
 	"fmt"
 	"io/ioutil"
 	"log"
@@ -149,6 +150,18 @@ func Supervise(commands []*exec.Cmd) error {
 		}
 	}()
 
+	go func() {
+		scanner := bufio.NewScanner(os.Stdin)
+		for scanner.Scan() {
+			if err := updateListeners("80"); err != nil {
+				log.Printf("updating listeners: %v", err)
+			}
+		}
+		if err := scanner.Err(); err != nil {
+			log.Print(err)
+		}
+	}()
+
 	return nil
 }