* vty: fix vty_dump_element(): do not print empty <attributes>
* vty/command: restrict the use of '.', '!', and '@' as flags
* vty/command: assign flags to CMD_ATTR_{IMMEDIATE,NODE_EXIT}
* vty: command.c: Fix: single-choice optional args are no longer passed incomplete to vty func
* vty: command.c: Fix: multi-choice args are no longer passed incomplete to vty func