MINOR: cli: always reset the applet task's timeout

MINOR: cli: always reset the applet task's timeout

The CLI applet doesn't make use of its timeout at all, only the stream
does. That's a wonder because it allows any command's I/O handler to
trivially set a wakeup timer by simply touching the task's ->expire
field, and the I/O handler will automatically be woken up again. The
only condition for this is that we properly take care of clearing that
timeout whenever we finish processing a command and switch back to the
PROMPT state. That's what this patch does.