MINOR: filters: Reject filter-* directives in anonymous defaults

MINOR: filters: Reject filter-* directives in anonymous defaults

Require a named defaults section for filter-config, filter-sequence and
filter-enable/filter-disable, consistently with the restriction on TCP/HTTP
rules. Reject these directives before parsing their arguments or recording
configuration entries. Named defaults and regular proxies remain supported.

Add regression coverage for each rejected directive and for valid usage in
named defaults, frontends and backends, including inherited instances.