[misc] refactor the "operation in progress" detection

* Remove unused iso_op_in_progress and use a single op_in_progress that gets
  set when we disable the controls.
* Also fix an issue where Ctrl-L was being processed as Alt-L due yet another
  completely backwards Windows behaviour where the message that is meant to
  indicating whether Alt is pressed is also sometimes used to indicate that
  another key is being pressed if the dialog doesn't have keyboard focus...

src/net.c

@@ -641,7 +641,7 @@ static DWORD WINAPI CheckForUpdatesThread(LPVOID param)
 		do {
 			for (i=0; (i<30) && (!force_update_check); i++)
 				Sleep(500);
-		} while ((!force_update_check) && ((iso_op_in_progress || format_op_in_progress || (dialog_showing>0))));
+		} while ((!force_update_check) && ((op_in_progress || (dialog_showing > 0))));
 		if (!force_update_check) {
 			if ((ReadSetting32(SETTING_UPDATE_INTERVAL) == -1)) {
 				vuprintf("Check for updates disabled, as per settings.");
@@ -829,7 +829,7 @@ static DWORD WINAPI CheckForUpdatesThread(LPVOID param)
 	// Start the new download after cleanup
 	if (found_new_version) {
 		// User may have started an operation while we were checking
-		while ((!force_update_check) && (iso_op_in_progress || format_op_in_progress || (dialog_showing > 0))) {
+		while ((!force_update_check) && (op_in_progress || (dialog_showing > 0))) {
 			Sleep(15000);
 		}
 		DownloadNewVersion();
@@ -994,7 +994,6 @@ static DWORD WINAPI DownloadISOThread(LPVOID param)
 			// Download the ISO and report errors if any
 			SendMessage(hMainDialog, UM_PROGRESS_INIT, 0, 0);
 			FormatStatus = 0;
-			format_op_in_progress = TRUE;
 			SendMessage(hMainDialog, UM_TIMER_START, 0, 0);
 			if (DownloadToFileOrBuffer(url, img_save.ImagePath, NULL, hMainDialog, TRUE) == 0) {
 				SendMessage(hMainDialog, UM_PROGRESS_EXIT, 0, 0);
@@ -1011,7 +1010,6 @@ static DWORD WINAPI DownloadISOThread(LPVOID param)
 				image_path = safe_strdup(img_save.ImagePath);
 				PostMessage(hMainDialog, UM_SELECT_ISO, 0, 0);
 			}
-			format_op_in_progress = FALSE;
 			safe_free(img_save.ImagePath);
 		}
 	}