MINOR: filters: add filter name to flt_conf struct

MINOR: filters: add filter name to flt_conf struct

flt_conf struct stores the filter id, which is used internally to check
match the filter against static pointer identifier, and also used as
descriptive text to describe the filter. But the id is not consistent
with the public name as used in the configuration (for instance when
selecting filter through the 'filter' directive).

What we do in this patch is that we add flt_conf->name member, which
stores the real filter name as seen in the configuration. This will
allow to select filters by their name from other directives in the
configuration.