cli: Fixed race condition. Split rxtx alloc/free routines up.

The corruption we were seeing was due to the RX task starting to
execute before the rxtx_data = rxtx_data_alloc() returned.
Therefore, the cli_state didn't yet have a valid rxtx_data pointer.

To address this, the rxtx data allocation has been split to
an allocation and a start_task, as it was poor design to have
allocation/init routines kicking off threads. There's a analagous
pair of routines for stopping the tasks before deinitializing.

Misc cleanup was also performed on buffer naming and general line
sizes and such.