Instead of a min term size, use a default (VPP-1061)

Instead of a min term size, use a default (VPP-1061)

- In the bug report, Docker was sometimes giving shells
  a 0,0 terminal size. The minimum-term-size logic meant
  that VPP assumed the terminal had 1 row. The pager
  functioned, but of course overwrote the one line with its
  own prompt.
- Instead of a minimum size, always use a default size when
  the either terminal dimension is 0.

Change-Id: Iee5a465f0e4cbb618ef2222b40a52994aefa54bf
Signed-off-by: Chris Luke <chrisy@flirble.org>