[CMD] Fix the errorlevel value set by the EXIT command, and when a batch file has run.

CORE-10495 CORE-13672

- Fix the behaviour of the EXIT command, where it set the last errorlevel
  value ONLY when it was called with the /b switch and otherwise kept the
  ambient one, instead of always using the value that the user specified
  on the command-line.

- When a batch file has terminated, make the Batch() helper returning the
  errorlevel value so that, when the batch file has been started with the
  CALL command, CALL can in turn set the correct errorlevel value.

Verified with respect to Windows' cmd.exe.

base/shell/cmd/batch.c

@@ -299,7 +299,8 @@ INT Batch (LPTSTR fullname, LPTSTR firstword, LPTSTR param, PARSED_COMMAND *Cmd)
          * return until this context has been exited */
         new.prev = bc;
         /* copy some fields in the new structure if it is the same file */
-        if (same_fn) {
+        if (same_fn)
+        {
             new.mem     = bc->mem;
             new.memsize = bc->memsize;
             new.mempos  = 0;
@@ -366,6 +367,9 @@ INT Batch (LPTSTR fullname, LPTSTR firstword, LPTSTR param, PARSED_COMMAND *Cmd)
         FreeCommand(Cmd);
     }
 
+    /* Always return the current errorlevel */
+    ret = nErrorLevel;
+
     TRACE ("Batch: returns TRUE\n");
 
     fc = saved_fc;