MINOR: filters: Set the class flags on filter class registration

MINOR: filters: Set the class flags on filter class registration

The classes registered with filter_register_class() or
filter_register_class_full() could not set their flags, in particular
FLT_CLS_FL_MULTI: the flags passed to flt_init_class() were always 0, so a
class of an external module was always limited to a single instance per
proxy, even with an id.

filter_register_class() and filter_register_class_full() now take a
'flags' argument, passed to flt_init_class() as for the internal classes.
The class flags are the FLT_CLS_FL_* flags, only FLT_CLS_FL_MULTI for now,
set when several instances of the class are allowed in the same proxy.

The filters documentation is updated accordingly, in the class
registration section and in the filter instance mode conversion guide.
The example in the comment of filter_init_classes() is updated too.