Multiple calls to the synthetic() function result in the accumulation of
all body parts. The ability to set a [be]resp body accidentally resulted
in the same behavior.

In other words writing `set resp.body = "string";` in VCL behaves as if
`set resp.body += "string";` was written instead. This patch introduces
an enum to distinguish between the two desired actions and enables both
syntaxes without touching the behavior of the synthetic() function.