Add a debug-CLI leak-checker

Add a debug-CLI leak-checker

leak-check { <any-debug-cli-command-and-args> }

Hint: "set term history off" or you'll have to sort through a bunch of
bogus leaks related to the debug cli history mechanism.

Cleaned up a set of reported leaks in the "show interface" command. At
some point, we thought about making a per-thread vlib_mains vector,
but we never did that. Several interface-related CLI's maintained
local static cache vectors. Not a bad idea, but not useful as things
shook out. Removed the static vectors.

Change-Id: I756bf2721a0d91993ecfded34c79da406f30a548
Signed-off-by: Dave Barach <dave@barachs.net>