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.

transactions.go

@@ -7,30 +7,43 @@ package go3270
 import "net"
 
 // Tx is a function that serves as one transaction in a go3270 application.
-// The Tx function is called with the network connection to the client, and a
-// "data" value provided by the previous transaction. Tx functions return the
-// next transaction to run (or nil to indicate the RunTransactions() function
-// should terminate), the data to pass into the next transaction, and any
-// error. If the error is non-nil, the RunTransactions() function will
-// terminate and return the err. A non-nil error is _not_ passed between
-// transactions, it terminates transaction processing.
-type Tx func(conn net.Conn, data any) (next Tx, newdata any, err error)
+// The Tx function is called with the network connection to the client, the
+// DevInfo for use with alternate screen writes, and a "data" value provided
+// by the previous transaction. Tx functions return the next transaction to
+// run (or nil to indicate the RunTransactions() function should terminate),
+// the data to pass into the next transaction, and any error. If the error is
+// non-nil, the RunTransactions() function will terminate and return the err.
+// A non-nil error is _not_ passed between transactions, it terminates
+// transaction processing.
+type Tx func(conn net.Conn, dev DevInfo, data any) (
+	next Tx, newdata any, err error)
 
 // RunTransactions begins running transaction functions, starting with the
 // initial transaction, until a transaction eventually returns nil for the
 // next transaction, or until a transaction function returns a non-nil error
 // value. data (which may be nil, if the initial transaction does not require
 // data) is passed in as the data to the initial transaction.
-func RunTransactions(conn net.Conn, initial Tx, data any) error {
+//
+// dev is the DevInfo of the connected client, as obtained from
+// NegotiateTelnet(). It is safe to pass a nil DevInfo, in which case all
+// transactions will only be able to operate with the default 24x80 screen
+// size.
+func RunTransactions(conn net.Conn, dev DevInfo, initial Tx,
+	data any) error {
+
 	var next Tx
 	var err error
 
 	next = initial
 
+	if dev == nil {
+		dev = &deviceInfo{rows: 24, cols: 80, termtype: "DEFAULT"}
+	}
+
 	// We run transactions until there isn't a next transaction to run, or
 	// an error.
 	for {
-		next, data, err = next(conn, data)
+		next, data, err = next(conn, dev, data)
 		if err != nil {
 			// Error means we bail out and return the error to the caller.
 			return err