[CMD] Additional fixes for ERRORLEVEL and last returned exit code from EXIT, CALL commands and CMD.

CORE-10495 CORE-13672

- Fix how the ERRORLEVEL and the last returned exit code are set by
  EXIT and CALL commands, when batch contexts terminate, and when CMD
  runs in single-command mode (with /C).

  Addendum to commit 26ff2c8e, and reverts commit 7bd33ac4.
  See also commit 8cf11060 (r40474).

  More information can be found at:
  https://ss64.com/nt/exit.html
  https://stackoverflow.com/a/34987886/13530036
  https://stackoverflow.com/a/34937706/13530036

- Move the actual execution of the CMD command-line (in /C or /K
  single-command mode) from Initialize() to _tmain(), to put it on par
  with the ProcessInput() interactive mode.

- Make ProcessInput() also return the last command's exit code.

base/shell/cmd/internal.c

@@ -548,13 +548,16 @@ INT CommandExit(LPTSTR param)
 
     /* Search for an optional exit code */
     while (_istspace(*param))
-        param++;
+        ++param;
 
     /* Set the errorlevel to the exit code */
     if (_istdigit(*param))
+    {
         nErrorLevel = _ttoi(param);
+        // if (fSingleCommand == 1) return nErrorLevel;
+    }
 
-    return 0;
+    return (bExit ? nErrorLevel : 0);
 }
 
 #ifdef INCLUDE_CMD_REM