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/example5.go

@@ -0,0 +1,48 @@
+// 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.
+
+// Example 5 demonstrates support for larger-than-default alternate screen
+// sizes in terminals that are larger than 24x80.
+
+package main
+
+import (
+	"fmt"
+	"net"
+
+	"github.com/racingmars/go3270"
+)
+
+func main() {
+	ln, err := net.Listen("tcp", ":3270")
+	if err != nil {
+		panic(err)
+	}
+	fmt.Println("LISTENING ON PORT 3270 FOR CONNECTIONS")
+	fmt.Println("Press Ctrl-C to end server.")
+	for {
+		conn, err := ln.Accept()
+		if err != nil {
+			panic(err)
+		}
+		go handle(conn)
+	}
+}
+
+// handle is the handler for individual user connections.
+func handle(conn net.Conn) {
+	defer conn.Close()
+
+	// Always begin new connection by negotiating the telnet options
+	devinfo, err := go3270.NegotiateTelnet(conn)
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+
+	err = go3270.RunTransactions(conn, devinfo, bigscreen, nil)
+	if err != nil {
+		fmt.Println(err)
+	}
+}