vlib: unix cli - log cli commands one per line

This changeset modifies the log output so that cli commands now look like:
  cat /tmp/vpp-cli.log
  2020/03/10 19:24:42:186: ***** Start: PID 29570 *****
  /* [0] is stdin */
  2020/03/10 19:24:53:518[0]: show clock
  2020/03/10 19:24:56:078[0]: show clock
  2020/03/10 19:25:01:983[0]: show pci
  /* [1] is vppctl socket */
  2020/03/10 19:26:14:372[1]: show ver
  2020/03/10 19:26:23:395[1]: show hardware-interfaces
  2020/03/10 19:26:28:507[1]: q

instead of:
  cat /tmp/vpp-cli.log
2020/03/01 00:40:12:834: ***** Start: PID 29974 *****
2020/03/01 00:40:24:969[0]: show interface2020/03/01 00:40:31:945[0]: show clo2020/03/01 00:40:41:288[0]: show classify 2020/03/01 00:40:43:753[0]: show classify ?2020/03/01 00:40:53:392[0]: show classify tables2020/03/01 00:40:57:352[0]: quit

Type: improvement

Change-Id: I38d85535977296d174db7121ae340850b6476a39
Signed-off-by: Paul Vinciguerra <pvinci@vinciconsulting.com>