lib: fix output mangling with | include

lib: fix output mangling with | include

Sometimes output would be mangled when filtering with include as a
result of the following bugs:

* Filters were applied per each call to vty_out() instead of buffering
  until a line break and then applying
* Long output would sometimes be cut due to using the wrong buffer
  pointer

Also remove the trailing \n as it should no longer be necessary to
ensure the vty prompt ends up on a new line.

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