MINOR: filters: initialize the filter classes before the register stage

MINOR: filters: initialize the filter classes before the register stage

The filter classes of HAProxy are initialized from the register stage,
where the internal classes are registered too. A module registering its
own class from that stage is refused, since whether it runs before or
after this initialization depends on the link order, and the objects of an
addon are linked first: its classes are registered too early, always.

Initializing the classes from the prepare stage leaves the whole register
stage to the modules, which then register their classes in time for
"haproxy -vv" to report them, the filter classes being listed while the
arguments are parsed, between the register and the init stages.

Nothing else moves: the classes are initialized in the same order, and the
placement of a class still decides the evaluation order of the filters.
The filter documentation was updated to match.