MINOR: acl: Warn when an ACL is named 'or'

MINOR: acl: Warn when an ACL is named 'or'

Consider a configuration like this:

> acl t always_true
> acl or always_false
>
> http-response set-header Foo Bar if t or t

The 'or' within the condition will be treated as a logical disjunction
and the header will be set, despite the ACL 'or' being falsy.

This patch makes it an error to declare such an ACL that will never
work. This patch may be backported to stable releases, turning the
error into a warning only (the code was written in a way to make this
trivial). It should not break anything and might improve the users'
lifes.