MINOR: debug: add CLI command "debug dev write" to write an arbitrary size

MINOR: debug: add CLI command "debug dev write" to write an arbitrary size

This command is used to produce an arbitrary amount of data on the
output. It can be used to test the CLI's state machine as well as
the internal parts related to applets an I/O. A typical test consists
in asking for all sizes from 0 to 16384:

  $ (echo "prompt;expert-mode on";for i in {0..16384}; do
     echo "debug dev write $i"; done) | socat - /tmp/sock1 | wc -c
  134258738

A better test would consist in first waiting for the response before
sending a new request.

This command is not restricted to the admin since it's harmless.