VPP-46 Fix cut-and-paste in the debug CLI

VPP-46 Fix cut-and-paste in the debug CLI

Since the move to line-mode the debug CLI was eating everything in the
input_vector but only processing upto the first newline. Cut-and-paste
type operations generally send a large block of input with multiple
newlines and thus all but the very first line were simply ignored.

This patch fixes that and also cleans up the difference between
input_vector and current_command which in turn removes a lot of cruft
from the keystroke parser.

Previously current_command was just the character accumulator inside the
char-by-char keystroke parser; complete commands were copied back to
input_vector (overwriting anything already in there).

Now, in char-by-char mode:
- input_vector is the stream of incoming bytes yet to be processed
- current_command is the accumulated characters of the next command to
  be executed; once newline is found, it is the complete command to be
  executed.

In line mode:
- input_vector and current_command are the same thing.

Change-Id: I72d21f0f3508b413879071ab186a71cef1124a2b
Signed-off-by: Chris Luke <chrisy@flirble.org>