During if_vmove() we call if_detach_internal() which in turn calls the event
handler notifying about interface departure and one of the consumers will
detach if_bpf.
There is no way for us to re-attach this easily as the DLT and hdrlen are
only given on interface creation.
Add a function to allow us to query the DLT and hdrlen from a current
BPF attachment and after if_attach_internal() manually re-add the if_bpf
attachment using these values.

Found by panics triggered by nd6 packets running past BPF_MTAP() with no
proper if_bpf pointer on the interface.

Also add a basic DDB show function to investigate the if_bpf attachment
of an interface.

Reviewed by: gnn
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D5896