Instead of triggering a WRONG() panic, we now fail the VCL when any vmod
function with PRIV_TOP argument(s) is present in a sub called from
outside client context.

Note that the VCL failure happens not when the vmod function with the
PRIV_TOP argument is called, but rather when the containing SUB is
called. This is because we prepare PRIV_TOP arguments in the function
preamble.

Fixes #3498