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.

example4/help.go

@@ -48,7 +48,8 @@ var helpScreen = go3270.Screen{
 // generator function for it is a stand-alone function and not a method on the
 // session.
 func help(returnTransaction go3270.Tx) go3270.Tx {
-	return func(conn net.Conn, data any) (go3270.Tx, any, error) {
+	return func(conn net.Conn, _ go3270.DevInfo, data any) (
+		go3270.Tx, any, error) {
 		_, err := go3270.HandleScreen(
 			helpScreen,                  // the screen to display
 			nil,                         // (no) rules to enforce