mac80211_hwsim driver support userspace frame tx/rx

This patch adds to mac80211_hwsim the capability to send traffic via
userspace.

Frame exchange between kernel and user spaces is done through generic
netlink communication protocol. A new generic netlink family
MAC80211_HWSIM is proposed, this family contains three basic commands
HWSIM_CMD_REGISTER, which is the command used to register a new
traffic listener, HWSIM_CMD_FRAME, to exchange the frames from kernel
to user and vice-versa, and HWSIM_CMD_TX_INFO_FRAME which returns
from user all the information about retransmissions, rates, rx signal,
and so on.

How it works:

Once the driver is loaded the MAC80211_HWSIM family will be registered.
In the absence of userspace daemon, the driver itselfs implements a
perfect wireless medium as it did in the past. When a daemon sends a
HWSIM_CMD_REGISTER command, the module stores the application PID, and
from this moment all frames will be sent to the registered daemon.

The user space application will be in charge of process/forward all
frames broadcast by any mac80211_hwsim radio. If the user application
is stopped, the kernel module will detect the release of the socket
and it will switch back to in-kernel perfect channel simulation.

The userspace daemon must be waiting for incoming HWSIM_CMD_FRAME
commands sent from kernel, for each HWSIM_CMD_FRAME command the
application will try to broadcast this frame to all mac80211_hwsim
radios, however the application may decide to forward/drop this frame.
In the case of forwarding the frame, a new HWSIM_CMD_FRAME command will
be created, all necessary attributes will be populated and the frame
will be sent back to the kernel.

Also after the frame broadcast phase, a HWSIM_CMD_TX_INFO_FRAME
command will be sent from userspace to kernel, this command contains
all the information regarding the transmission, such as number of
tries, rates, ack signal, etc.

You can find the actual implementation of wireless mediumd daemon
(wmediumd) at:

* Last version tarball: https://github.com/jlopex/cozybit/tarball/master
* Or visiting my github tree: https://github.com/jlopex/cozybit/tree

Signed-off-by: Javier Lopez <jlopex@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

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