Add special parsing of ENUM in vmod object method argument spec

The missing handler would end the object spec prematurily, hiding the
rest of the methods.

Fixes: #1332
Stash the predictive Vary: string in malloc space, we need to get
the req workspace un-reserved before we can call vcl_lookup{} and
we can't tell if we need the predictive Vary: until vcl_lookup{}
tells us.
Ever so slightly closer to The New World Order