MINOR: vars: make vars_get_by_* support an optional default value

MINOR: vars: make vars_get_by_* support an optional default value

In preparation for support default values when fetching variables, we
need to update the internal API to pass an extra argument to functions
vars_get_by_{name,desc} to provide an optional default value. This
patch does this and always passes NULL in this argument. var_to_smp()
was extended to fall back to this value when available.