Handle keypad mode cursor key escape sequences. (#203)

Normally the terminal uses CSI escape sequences when the UP, DOWN,
LEFT, RIGHT and HOME, END keys are pressed. These look like the
following ESC [ A etc, where ESC [ is the CSI sequence.

xterm and other terminals however can generate an alternative
escape sequence called SS3 if in the application keypad mode.
This sequence is ESC O A etc.

Bash readline understands both modes so nowadays you rarely
see OA being printed when you press the up arrow while the terminal
is using the keypad mode. readline currently does not understand
these sequences.

To test this fix, I used an xterm and put it in keypad mode
using the command "tput smkx". Then I started the readline-demo
and tried using arrow keys. Without this fix, OA is printed when
I press up. With this fix, readline fetches the previous command
as per regular mode. After testing you can escape back to
regular mode using "tput rmkx".

terminal.go

@@ -125,6 +125,7 @@ func (t *Terminal) ioloop() {
 	var (
 		isEscape       bool
 		isEscapeEx     bool
+		isEscapeSS3    bool
 		expectNextChar bool
 	)
 
@@ -152,9 +153,15 @@ func (t *Terminal) ioloop() {
 		if isEscape {
 			isEscape = false
 			if r == CharEscapeEx {
+				// ^][
 				expectNextChar = true
 				isEscapeEx = true
 				continue
+			} else if r == CharO {
+				// ^]O
+				expectNextChar = true
+				isEscapeSS3 = true
+				continue
 			}
 			r = escapeKey(r, buf)
 		} else if isEscapeEx {
@@ -177,6 +184,15 @@ func (t *Terminal) ioloop() {
 				expectNextChar = true
 				continue
 			}
+		} else if isEscapeSS3 {
+			isEscapeSS3 = false
+			if key := readEscKey(r, buf); key != nil {
+				r = escapeSS3Key(key)
+			}
+			if r == 0 {
+				expectNextChar = true
+				continue
+			}
 		}
 
 		expectNextChar = true

utils.go

@@ -43,6 +43,7 @@ const (
 	CharCtrlY     = 25
 	CharCtrlZ     = 26
 	CharEsc       = 27
+	CharO         = 79
 	CharEscapeEx  = 91
 	CharBackspace = 127
 )
@@ -123,6 +124,27 @@ func escapeExKey(key *escapeKeyPair) rune {
 	return r
 }
 
+// translate EscOX SS3 codes for up/down/etc.
+func escapeSS3Key(key *escapeKeyPair) rune {
+	var r rune
+	switch key.typ {
+	case 'D':
+		r = CharBackward
+	case 'C':
+		r = CharForward
+	case 'A':
+		r = CharPrev
+	case 'B':
+		r = CharNext
+	case 'H':
+		r = CharLineStart
+	case 'F':
+		r = CharLineEnd
+	default:
+	}
+	return r
+}
+
 type escapeKeyPair struct {
 	attr string
 	typ  rune