if_bridge(4) modifications refactored completely.

It is possible filter rfc2516 based frame containing IPv[46]
SDU by hooked packet filtering facility. Frames are reassembled
as frames with protocol id ETHERTYPE_IP[6]. Further, during
runtime of bridge_dummynet() original protocol id and PCI
relating rfc-2516 and rfc-1661 are restored, if MPI is annotated by

	struct m_tag_pppoe {
		struct m_tag	mtp_tag;
		struct pppoe_hdr	mtp_pci;
	};
	#define	MTAG_PPPOE		1402876965
	#define	MTAG_PPPOE_PCI	0