Alternate screen size support.

This is a relatively big new feature, which introduces small breaking changes
to the API:

 * NegotiateTelnet() now returns (DevInfo, error) instead of just error.  The
   function signature for the Tx type now has one additional DevInfo parameter.
 * RunTransactions() requires one additional parameter, the DevInfo (which is
   allowed to be nil).

Otherwise, existing behavior continues to work as-is and everything still
defaults to operating on the default 24x80 terminal size.

See example5 for a larger-terminal-aware application.

Pending some time to make sure no new bugs cropped up in this release and that
the alternate screen size support works in the wild, I anticipate this will
become the v1.0.0 release of go3270.

bufaddr_test.go

@@ -9,15 +9,21 @@ import (
 )
 
 func TestEncode(t *testing.T) {
-	encoded := getpos(0, 0)
+	encoded := getpos(0, 0, 80)
 	if encoded[0] != 0x40 || encoded[1] != 0x40 {
 		t.Error("Position (0, 0) not correctly encoded")
 	}
 
-	encoded = getpos(11, 39)
+	encoded = getpos(11, 39, 80)
 	if encoded[0] != 0x4e || encoded[1] != 0xd7 {
 		t.Error("Position (11, 39) not correctly encoded")
 	}
+
+	// Large screen, 14-bit addressing
+	encoded = getpos(100, 120, 130)
+	if encoded[0] != 0x33 || encoded[1] != 0x40 {
+		t.Errorf("Position (100, 120) on 130-col screen not correctly encoded")
+	}
 }
 
 func TestDecode(t *testing.T) {
@@ -30,4 +36,10 @@ func TestDecode(t *testing.T) {
 	if decoded != 919 {
 		t.Error("Buffer address incorrectly decoded")
 	}
+
+	// Large screen, 14-bit addressing
+	decoded = decodeBufAddr([2]byte{0x33, 0x40})
+	if decoded != 13120 {
+		t.Error("14-bit buffer address incorrectly decoded")
+	}
 }