BUG/MEDIUM: cli: Don't consume data if outbuf is full or not available

BUG/MEDIUM: cli: Don't consume data if outbuf is full or not available

The output buffer must be available to process a command, at least to be
able to emit error messages. When this buffer is full or cannot be
allocated, we must wait. In that case, we must take care to notify the SE
will not consume input data. It is important to avoid wakeup in loop,
especially when the client aborts.

When the output buffer is available again and no longer full, and the CLI
applet is waiting for a command line, it must notify it will consume input
data.

This patch must be backported as far as 3.0.