add a timeout between capturing a packet and making the buffer readable.

add a timeout between capturing a packet and making the buffer readable.

before this, there were three reasons that a bpf read will finish.

the first is the obvious one: the bpf packet buffer in the kernel
fills up. by default this is about 32k, so if you're only capturing
a small packet packet every few seconds, it can take a long time
for the buffer to fill up before you can read them.

the second is if bpf has been configured to enable immediate mode with
ioctl(BIOCIMMEDIATE). this means that when any packet is written into
the bpf buffer, the buffer is immediately readable. this is fine
if the packet rate is low, but if the packet rate is high you don't
get the benefit of buffering many packets that bpf is supposed to
provide.

the third mechanism is if bpf has been configured with the BIOCSRTIMEOUT
ioctl, which sets a maximum wait time on a bpf read. BIOCSRTIMEOUT
means than a clock starts ticking down when a program (eg pflogd)
reads from bpf. when the clock reaches zero then the read returns
with whatever is in the bpf packet buffer. however, there could be
nothing in the buffer, and the read will still complete.

deraadt@ noticed this behaviour with pflogd. it wants packets logged
by pf to end up on disk in a timely fashion, but it's fine with
tolerating a bit of delay so it can take advantatage of buffering
to amortise the cost of the reads per packet. it currently does
this with BIOCSRTIMEOUT set to half a second, which means it's
always waking up every half second even if there's nothing to log.

this diff adds BIOCSWTIMEOUT, which specifies a timeout from when
bpf first puts a packet in the capture buffer, and when the buffer
becomes readable.

by default this wait timeout is infinite, meaning the buffer has
to be filled before it becomes readable. BIOCSWTIMEOUT can be set
to enable the new functionality. BIOCIMMEDIATE is turned into a
variation of BIOCSWTIMEOUT with the wait time set to 0, ie, wait 0
seconds between when a packet is written to the buffer and when the
buffer becomes readable. combining BIOCSWTIMEOUT and
BIOCIMMEDIATE simplifies the code a lot.

for pflogd, this means if there are no packets to capture, pflogd
won't wake up every half second to do nothing.  however, when a
packet is logged by pf, bpf will wait another half second to see
if any more packets arrive (or the buffer fills up) before the read
fires.

discussed a lot with deraadt@ and sashan@
ok sashan@