add a MONITOR flag to ifaces to say they're only used for watching packets.

add a MONITOR flag to ifaces to say they're only used for watching packets.

an example use of this is when you have a span port on a switch and
you want to be able to see the packets coming out of it with tcpdump,
but do not want these packets to enter the network stack for
processing. this is particularly important if the span port is
pushing a copy of any packets related to the machine doing the
monitoring as it will confuse pf states and the stack.

ok benno@