vtysh: optimize printout routine

When printing responses from a client, vtysh searches through every
response it receives twice. Once is to look for the terminating
sequence and the other is to translate \n to \0, which is used with the
line processing callback capability. However:

* There's no need to search all of the output for the terminator, we can
  just check the last 4 bytes.
* In most scenarios we won't have a callback. Therefore we don't need to
  process the output and can just dump it.

Together these optimizations have a significant impact on the total
runtime of talkative CLI commands, such as `sh ip bgp json` which runs
roughly 1000% faster when dumping 1,000,000 routes.

Signed-off-by: Quentin Young <qlyoung@cumulusnetworks.com>

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