go.crypto/ssh/terminal: support Unicode entry.

Previously, terminal only supported ASCII characters. This change
alters some []byte to []rune so that the full range of Unicode is
supported. The only thing that doesn't appear to work correctly are
grapheme clusters as the code still assumes one rune per glyph. Still,
this change allows many more languages to work than did previously.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/13704043

ssh/terminal/terminal_test.go

@@ -137,6 +137,10 @@ var keyPressTests = []struct {
 		in:   "ab\x1b[D\013\r",
 		line: "a",
 	},
+	{
+		in:   "Ξεσκεπάζω\r",
+		line: "Ξεσκεπάζω",
+	},
 }
 
 func TestKeyPresses(t *testing.T) {