Mainly modify BC_Term_Menus, BC_NetworkAdmin and add BC_OS_Utilit

1. Add following attributes to BC_Term_Menus.pm:
clear_screen, multi_menu_item, prt_control
2. Add initial version of BC_OS_Utilit.pm.
3. Modify BC_NetworkAdmin.pm:
1) Add ftpobj, ftp_establis_session attribute
2) Add ftp_login, ftp_close instant method.