Mainly changes: prevent overwriting existed file that we want to download

1. Modify BC_NetworkAdmin.pm to prevent overwriting existed file that we
want to download.
2. Fix a bug in BC_Term_Menus.pm. If without fix, it will add redundant
bullets to menu items when displaying menu after first time.
3. Add flow to allow quit application and back to up menu in
RemoteExcelMgt.pl