Handle symbols are ACL, BACKEND, PROBE and STEVEDORE, they are not
variables, but "things which do stuff".

The only real difference from variables (like resp.status) is that
when they are stringified (resp.http.foo = BLA) they return the
name of the thing (ie: backend, acl ... name) rather than a value,
which they don't have.

Handles are by definition available everywhere.

Global symbols are handles available in any VCL (ie: STEVEDOREs).