rl_helper: add support to tab completer

We can pass a simple handler to complete words when tab is pressed.
Add simple tab completer

Test start of line if match some command and return first one, if has.
Fix wrong prompt_state/r_bd_addr placement

The prompt_state and the r_bd_addr variables were wrongly placed as
global, but we can put them inside userdate structure.
As a side effect, we need to initialize prompt_state calling
change_prompt_state with NORMAL_PSTATE parameter.