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.

util.go

@@ -23,8 +23,8 @@ func debugf(format string, a ...interface{}) {
 	fmt.Fprintf(Debug, format, a...)
 }
 
-// codes are the 3270 control character I/O codes, pre-computed as provided
-// at http://www.tommysprinkle.com/mvs/P3270/iocodes.htm
+// codes are the 3270 control character I/O codes for 12-bit addressing,
+// from Figure D-1 of GA23-0059-00. (Figure C-1 in later editions.)
 var codes = []byte{0x40, 0xc1, 0xc2, 0xc3, 0xc4, 0xc5, 0xc6, 0xc7, 0xc8,
 	0xc9, 0x4a, 0x4b, 0x4c, 0x4d, 0x4e, 0x4f, 0x50, 0xd1, 0xd2, 0xd3, 0xd4,
 	0xd5, 0xd6, 0xd7, 0xd8, 0xd9, 0x5a, 0x5b, 0x5c, 0x5d, 0x5e, 0x5f, 0x60,
@@ -32,23 +32,6 @@ var codes = []byte{0x40, 0xc1, 0xc2, 0xc3, 0xc4, 0xc5, 0xc6, 0xc7, 0xc8,
 	0x6d, 0x6e, 0x6f, 0xf0, 0xf1, 0xf2, 0xf3, 0xf4, 0xf5, 0xf6, 0xf7, 0xf8,
 	0xf9, 0x7a, 0x7b, 0x7c, 0x7d, 0x7e, 0x7f}
 
-// decodes is the inverse of the above table; -1 is used in invalid positions
-var decodes = []int{-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
-	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
-	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
-	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 0, -1, -1, -1,
-	-1, -1, -1, -1, -1, -1, 10, 11, 12, 13, 14, 15, 16, -1, -1, -1, -1, -1,
-	-1, -1, -1, -1, 26, 27, 28, 29, 30, 31, 32, 33, -1, -1, -1, -1, -1, -1,
-	-1, -1, 42, 43, 44, 45, 46, 47, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
-	58, 59, 60, 61, 62, 63, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
-	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
-	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
-	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 1, 2,
-	3, 4, 5, 6, 7, 8, 9, -1, -1, -1, -1, -1, -1, -1, 17, 18, 19, 20, 21, 22,
-	23, 24, 25, -1, -1, -1, -1, -1, -1, -1, -1, 34, 35, 36, 37, 38, 39, 40,
-	41, -1, -1, -1, -1, -1, -1, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, -1,
-	-1, -1, -1, -1}
-
 // AIDtoString returns a string representation of an AID key name.
 func AIDtoString(aid AID) string {
 	switch aid {