dpif: Simplify the "listen mask" concept.

At one point in the past, there were three separate queues between the
kernel module and OVS userspace, each of which corresponded to a Netlink
socket (or, before that, to a character device).  It made sense to allow
each of these to be enabled or disabled separately, hence the "listen mask"
concept in the dpif layer.

These days, the concept is much less clear-cut.  Queuing is no longer on
the basis of different classes of packets but instead striped across a
collection of sockets based on input port.  It doesn't really make sense
to enable receiving packets on the basis of the kind of packet anymore.
Accordingly, this commit simplifies the "listen_mask" to just a bool that
either enables or disables receiving packets.

It could be useful to enable or disable receiving packets on a per-vport
basis, but the rest of the code isn't ready to make use of that so this
commit doesn't generalize this much.

Based on this discussion on ovs-dev:
http://openvswitch.org/pipermail/dev/2011-October/012044.html

Signed-off-by: Ben Pfaff <blp@nicira.com>