MINOR: filters: List the filter classes in haproxy -vv

MINOR: filters: List the filter classes in haproxy -vv

The "Available filters" section dumped the keywords registered for the
legacy "filter" directive, one per line with the scope of the keyword
list they come from. New filters will support the new declaration mode
only and will not register such keywords, so the list would be
incomplete. And the printed names are the legacy keyword names, which
are not the filter class names usable with "filter-config" anyway.

Dump the registered filter classes instead, as a comma-separated list,
in their global order, so both declaration modes are covered. Rename
list_filters() to list_filter_classes() and change the header of the
dump accordingly. The legacy keyword list remains reported by
flt_dump_kws(), where a keyword list is what is expected: the "filter"
keyword parsing errors and the "haproxy -dM flt" diagnostics.

Update the "haproxy -vv" example in the internal API documentation and
the sample debug outputs in the regression testing documentation.