Add separate binary 'vnm_validate'

Re-uses same code as the vmod itself, validates that a given
JSON input file parses and loads successfully on the commandline
as a pre-check to be executed before updating the vmod's config.

Change-Id: Id9f5e5b45bcd90b933f75a97d621a7c2a1deb537
add cmdline debugging in vnm_validate

Change-Id: I7ef47eb86472b99a47e9a94434d847ffaa6f569c