MINOR: filters: Deduce the default filter API from the implied filters

MINOR: filters: Deduce the default filter API from the implied filters

Until now, a proxy with no explicit filter declaration (neither "filter" nor
"filter-*" directives) always used the new filter API, except when the
legacy API was globally forced with "filter-api downgrade-to-legacy". But a
third-party filter relying on an implicit declaration only, and not
registering a filter class, was silently ignored in the new API: such a
filter declares a legacy filter configuration, without any class, hence
without any instance.

Now the API used by a proxy without explicit filter declaration is deduced
from what its other directives imply. A proxy with filter configurations and
no filter instance uses the legacy API: the implied filters declared no
class, so they are invisible to the new API. A proxy with filter instances
uses the new API, where the class-less filters it may also imply are
ignored. The explicit declarations and the "filter-api downgrade-to-legacy"
option keep precedence, as before.

All the filters supported in-tree register their classes and are
instantiated in the new API, so they are unaffected: a proxy implying only
in-tree filters always has instances, and keeps using the new API.  The
change only makes the class-less third-party filters work again without the
global downgrade, and it is the first step towards making the new API the
only one.

The documentation is updated accordingly, in the filters internals guide and
in the configuration manual, for the "filter-api" option and the filter mode
selection.