go.crypto/ssh/terminal: support Go 1.0.

For those still stuck on Go 1.0.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/11297043

terminal.go

@@ -568,6 +568,8 @@ func (t *Terminal) readLine() (line string, err error) {
 
 		t.remainder = t.inBuf[:n+len(t.remainder)]
 	}
+
+	panic("unreachable") // for Go 1.0.
 }
 
 // SetPrompt sets the prompt to be used when reading subsequent lines.