Non-screen-clearing update support

New ShowScreenOpts() function, which takes a new ScreenOpts struct, to control
various aspects of the ShowScreen behavior. This should now be used in favor of
the deprecated ShowScreen() and ShowScreenNoResponse() functions, and allows
for the additional of future options without needing to continue making new
functions or breaking the public API.

looper.go

@@ -69,18 +69,18 @@ type FieldRules struct {
 // HandleScreen will loop until all validation rules are satisfied, and only
 // return when an expected AID (i.e. PF) key is pressed.
 //
-//  - screen is the Screen to display (see ShowScreen()).
-//  - rules are the Rules to enforce: each key in the Rules map corresponds to
-//    a Field.Name in the screen array.
-//  - values are field values you wish to override (see ShowScreen()).
-//  - pfkeys and exitkeys are the AID keys that you wish to accept (that is,
-//    perform validation and return if successful) and treat as exit keys
-//    (unconditionally return).
-//  - errorField is the name of a field in the screen array that you wish error
-//    messages to be written in when HandleScreen loops waiting for a valid
-//    user submission.
-//  - crow and ccol are the initial cursor position.
-//  - conn is the network connection to the 3270 client.
+//   - screen is the Screen to display (see ShowScreen()).
+//   - rules are the Rules to enforce: each key in the Rules map corresponds to
+//     a Field.Name in the screen array.
+//   - values are field values you wish to override (see ShowScreen()).
+//   - pfkeys and exitkeys are the AID keys that you wish to accept (that is,
+//     perform validation and return if successful) and treat as exit keys
+//     (unconditionally return).
+//   - errorField is the name of a field in the screen array that you wish error
+//     messages to be written in when HandleScreen loops waiting for a valid
+//     user submission.
+//   - crow and ccol are the initial cursor position.
+//   - conn is the network connection to the 3270 client.
 //
 // HandleScreen will return when the user: 1) presses a key in pfkeys AND all
 // fields pass validation, OR 2) the user presses a key in exitkeys. In all
@@ -127,7 +127,8 @@ mainloop:
 			}
 		}
 
-		resp, err := ShowScreen(screen, myValues, crow, ccol, conn)
+		resp, err := ShowScreenOpts(screen, myValues, conn,
+			ScreenOpts{CursorRow: crow, CursorCol: ccol})
 		if err != nil {
 			return resp, err
 		}