BUG/MEDIUM: cache: forbid cache rules in defaults sections

BUG/MEDIUM: cache: forbid cache rules in defaults sections

A "cache-use" or "cache-store" rule declared in a defaults section has
never worked: the implicit cache-store filter it creates is attached to
the defaults section itself, where the "filter" directive is not
supported. The filter is never resolved, and the rule inherited by the
proxies references a configuration pointing at an unresolved cache name
instead of a cache, which leads to crashes or silently broken caching
depending on the filter mode of the proxies.

Supporting these rules with the legacy filters would require a deep
rework of the rule-to-filter binding, hard to backport. So, until the
new filter API covers this case, the rules are simply forbidden in a
defaults section, with an explicit alert:

    [ALERT] : Proxy 'd': 'cache-use' rules are not supported in a defaults section.

This patch must be backported to all supported versions.