Move the definition of struct bpf_if to bpf.c.

[markj]

Summary

Fields needed for bpf_peers_present() are kept in a struct bpf_if_ext at
the beginning of struct bpf_if, so bpf_peers_present() effectively works
the same as before. While here, rename the flags field to bif_flags.

[markj]

• Make sure struct bpf_if's definition is available for bp_null.

[melifaro]

Well, while this change doesn't eliminate bpf_peers_present() hack, it makes it feel a bit less ugly.
(One of options to do this is to make per-interface PFIL hook and attach all bridge,netgraph,vlan and BPF hooks there)

[markj]

Closed by commit rS281797 (authored by @markj).