Allow the BPF to be select for EVFILT_WRITE with kqueue.

[harti]

Summary

Allow the BPF to be select for EVFILT_WRITE with kqueue. This is required for Boost asio to be able to use a BPF file descriptor even if it is used only for reading. Asio insists on installing a write filter.

[ae]

sys/net/bpf.c

2193

I'd prefer a bit less code duplication:

struct filterops *ops;

...

switch (kn->kn_filter) {
case EVFILT_READ:
   ops = &bpfread_filtops;
   break;
case EVFILT_WRITE:
   ops = &bpwrite_filtops;
   break;
default:
   return (EINVAL);
}

...

Also it seems you need to modify kqueue(2).

[harti]

Reduce code duplication.
Update kqueue.2.