if bpf fd's select()able?

[Guy Harris]

> However, my tests show that select()ing bpf fd does not lead to 
> trigger packets
> available to bpf filter; the process hangs in select state while 
> parallel
> tcpdump process shows packets desired *and* is in bpf state.

See PR kern/22063 "bpf when used with the select system call with 
timeout doesn't forward packets on timeout".

That *should* work in 4-STABLE, as that fix is in 4-STABLE.  You should 
not have to turn BIOCIMMEDIATE on (tcpdump doesn't).