MAJOR: cli: Use a custom .snd_buf function to only copy the current command

MAJOR: cli: Use a custom .snd_buf function to only copy the current command

The CLI applet is now using its own snd_buf callback function. Instead of
copying as most output data as possible, only one command is copied at a
time.

To do so, a new state CLI_ST_PARSEREQ is added for the CLI applet. In this
state, the CLI I/O handle knows a full command was copied into its input
buffer and it must parse this command to evaluate it.