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.

example5/bigscreen.go

@@ -0,0 +1,124 @@
+// This file is part of https://github.com/racingmars/go3270/
+// Copyright 2025 by Matthew R. Wilson, licensed under the MIT license. See
+// LICENSE in the project root for license information.
+
+package main
+
+import (
+	"fmt"
+	"net"
+	"strconv"
+
+	"github.com/racingmars/go3270"
+)
+
+var biglayout = go3270.Screen{
+	// Column will be calculated at runtime for the following field:
+	{Row: 0, Intense: true, Content: "3270 Screen Size Example"},
+
+	{Row: 2, Col: 0,
+		Content: "This screen is using the full size that your terminal supports."},
+
+	{Row: 4, Col: 0, Content: "Terminal Type  . . ."},
+	{Row: 4, Col: 21, Name: "termtype", Intense: true},
+
+	{Row: 5, Col: 0, Content: "Rows . . . . . . . ."},
+	{Row: 5, Col: 21, Name: "rows", Intense: true},
+
+	{Row: 6, Col: 0, Content: "Columns  . . . . . ."},
+	{Row: 6, Col: 21, Name: "cols", Intense: true},
+
+	{Row: 8, Col: 0, Content: "To visit a default sized screen, press"},
+	{Row: 8, Col: 39, Content: "PF1", Color: go3270.Yellow, Intense: true},
+
+	{Row: 9, Col: 0, Content: "To exit and disconnect, press"},
+	{Row: 9, Col: 30, Content: "PF3", Color: go3270.Yellow, Intense: true},
+
+	// a blank field for error messages
+	{Row: 11, Col: 0, Intense: true, Color: go3270.Red, Name: "errormsg"},
+}
+
+func bigscreen(conn net.Conn, devinfo go3270.DevInfo, data any) (
+	go3270.Tx, any, error) {
+
+	rows, cols := devinfo.AltDimensions()
+	termtype := devinfo.TerminalType()
+
+	// Make a local copy of the screen definition that we can append lines to.
+	screen := make(go3270.Screen, len(biglayout))
+	copy(screen, biglayout)
+
+	// Center the title on any screen width
+	screen[0].Col = (cols / 2) - (len(biglayout[0].Content) / 2)
+
+	// We'll start writing "data lines" at row 13 up to the penultimate row on
+	// the terminal
+	for i := 13; i < rows-1; i++ {
+		newfield := go3270.Field{Row: i, Col: 0,
+			Content: fmt.Sprintf("This is data row %d.", i-12)}
+		if i == rows-2 {
+			newfield.Content += " (The last.)"
+		}
+		screen = append(screen, newfield)
+
+		// And demonstrate that we can position to the full width, too.
+		newfield = go3270.Field{Row: i, Col: cols - 5, Content: "<**>"}
+		screen = append(screen, newfield)
+	}
+
+	// And an input field on the last row, to make sure field buffer address
+	// decoding works on larger screens.
+	newfield := go3270.Field{Row: rows - 1, Col: 0,
+		Content: "Enter data here:", Color: go3270.Pink, Intense: true}
+	screen = append(screen, newfield)
+	newfield = go3270.Field{Row: rows - 1, Col: 17, Name: "inputdata",
+		Write: true}
+	screen = append(screen, newfield)
+	newfield = go3270.Field{Row: rows - 1, Col: cols - 1} // "stop" field
+	screen = append(screen, newfield)
+
+	fieldValues := map[string]string{
+		"termtype": termtype,
+		"rows":     strconv.Itoa(rows),
+		"cols":     strconv.Itoa(cols),
+	}
+
+	if data != nil {
+		fieldValues["errormsg"] = fmt.Sprintf("You said: %s", data.(string))
+	}
+
+	resp, err := go3270.HandleScreenAlt(
+		screen,      // the screen to display
+		nil,         // (no) rules to enforce
+		fieldValues, // pre-populated values in fields
+		[]go3270.AID{ // keys we accept -- validating
+			go3270.AIDEnter,
+		},
+		[]go3270.AID{ // keys we accept -- non-validating
+			go3270.AIDPF1,
+			go3270.AIDPF3,
+		},
+		"errormsg", // name of field to put error messages in
+		rows-1, 18, // cursor coordinates
+		conn,    // network connection
+		devinfo, // device info for alternate screen size support
+	)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	switch resp.AID {
+	case go3270.AIDEnter:
+		// Re-run current transaction, echoing back input
+		return bigscreen, resp.Values["inputdata"], err
+	case go3270.AIDPF1:
+		// Go to default screen size transaction
+		return normalscreen, nil, nil
+	case go3270.AIDPF3:
+		// Exit
+		return nil, nil, nil
+	default:
+		// re-run current transaction
+		return bigscreen, nil, nil
+	}
+}