This commit introduce a typed virtual table for operations on demux,
aiming at replacing the legacy pf_control() callback which is using
va_list. As a counterpart to the typed virtual table, typed methods
are also introduce.

The operations can be implemented by the modules directly right now
and will be used when possible. This is done to provide type safety
at every level.

When no operation is provided (ie, demux_t.ops is NULL) by a module
implementation, the legacy pf_control will be used instead as a
fallback.

The commit doesn't migrate any of modules yet.