Fix panic on attaching to non-existent interface (introduced by r233937, pointed by hrs@)
Fix panic on tcpdump being attached to interface being removed (introduced by r233937, pointed by hrs@ and adrian@)
Protect most of bpf_setf() by BPF global lock

Add several forgotten assertions (thanks to adrian@)

Document current locking model inside bpf.c
Document EVENTHANDLER(9) usage inside BPF.

Approved by: kib(mentor)
Tested by: gnn
MFC in: 4 weeks