Have example sftp servers handle session EOF

As the EOF is expected when the client quits the session, have the
example servers handle it more gracefully so it is less confusing.

Fixes #169

examples/request-server/main.go

@@ -6,6 +6,7 @@ package main
 import (
 	"flag"
 	"fmt"
+	"io"
 	"io/ioutil"
 	"log"
 	"net"
@@ -120,7 +121,10 @@ func main() {
 
 		root := sftp.InMemHandler()
 		server := sftp.NewRequestServer(channel, root)
-		if err := server.Serve(); err != nil {
+		if err := server.Serve(); err == io.EOF {
+			server.Close()
+			log.Print("sftp client exited session.")
+		} else if err != nil {
 			log.Fatal("sftp server completed with error:", err)
 		}
 	}

examples/sftp-server/main.go

@@ -6,6 +6,7 @@ package main
 import (
 	"flag"
 	"fmt"
+	"io"
 	"io/ioutil"
 	"log"
 	"net"
@@ -136,7 +137,10 @@ func main() {
 		if err != nil {
 			log.Fatal(err)
 		}
-		if err := server.Serve(); err != nil {
+		if err := server.Serve(); err == io.EOF {
+			server.Close()
+			log.Print("sftp client exited session.")
+		} else if err != nil {
 			log.Fatal("sftp server completed with error:", err)
 		}
 	}