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/example4.go

@@ -75,8 +75,13 @@ func handle(conn net.Conn, db DB, gblstate *global) {
 	}()
 
 	// Always begin new connection by negotiating the telnet options
-	go3270.NegotiateTelnet(conn)
-	err := go3270.RunTransactions(conn, state.login, nil)
+	devinfo, err := go3270.NegotiateTelnet(conn)
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+
+	err = go3270.RunTransactions(conn, devinfo, state.login, nil)
 	if err != nil {
 		fmt.Println(err)
 	}