proposal part 1 - VCL: edit header tokens/entities

[Nils Goroll]

Status on this from https://www.varnish-cache.org/trac/wiki/VDD14Q3 :

We might want to write or at least prototype a vmod for it, but nothing is
pushing me personally to do this with high priority.

The main motivation for my proposal was that we probably need more token/entity
editing in core code anyway and so it was just an obvious idea to add the
functionality to VCC properly.

Nils