[CMD] Fixes for Batch error execution control flow.

CORE-13713 CORE-13736

- In case execution of all batch contexts is stopped (by selecting "All"
  at the Ctrl-C/Ctrl-Break prompt), notify as well the CheckCtrlBreak()
  signal handler once there are no more batch contexts (this in effect
  resets the internal 'bLeaveAll' static flag in CheckCtrlBreak).
  This is an adaptation of the fix present in FreeCOM 1.5, first
  described in https://gcfl.net/FreeDOS/command.com/bugs074g.html .

- Introduce a ParseErrorEx() helper that sets the 'bParseError' flag and
  displays a customized syntax-error message, only for the first syntax
  error encountered. Implement ParseError() around the *Ex function.

- In batch mode, echo the original pre-parsed batch file line if a parse
  error has been encountered.

- When running a compound command - including IF, FOR, command blocks -,
  and that control flow is modified by any CALL/GOTO/EXIT command,
  detect this while running the compound command so as to stop it and go
  back to the main batch execution loop, that will then set up the actual
  new command to run.

- In GOTO, do not process any more parts of a compound command only when
  we have found a valid label.

base/shell/cmd/parser.c

@@ -60,9 +60,9 @@ enum
 /* Scratch buffer for temporary command substitutions / expansions */
 static TCHAR TempBuf[CMDLINE_LENGTH];
 
-static BOOL bParseError;
+/*static*/ BOOL bParseError;
 static BOOL bLineContinuations;
-static TCHAR ParseLine[CMDLINE_LENGTH];
+/*static*/ TCHAR ParseLine[CMDLINE_LENGTH];
 static TCHAR *ParsePos;
 static TCHAR CurChar;
 
@@ -108,10 +108,17 @@ restart:
     return (CurChar = Char);
 }
 
+void ParseErrorEx(LPTSTR s)
+{
+    /* Only display the first error we encounter */
+    if (!bParseError)
+        error_syntax(s);
+    bParseError = TRUE;
+}
+
 static void ParseError(void)
 {
-    error_syntax(CurrentTokenType != TOK_END ? CurrentToken : NULL);
-    bParseError = TRUE;
+    ParseErrorEx(CurrentTokenType != TOK_END ? CurrentToken : NULL);
 }
 
 /*