Add support for VALE bridges to the netmap core, see

http://info.iet.unipi.it/~luigi/vale/

VALE lets you dynamically instantiate multiple software bridges
that talk the netmap API (and are *extremely* fast), so you can test
netmap applications without the need for high end hardware.

This is particularly useful as I am completing a netmap-aware
version of ipfw, and VALE provides an excellent testing platform.

Also, I also have netmap backends for qemu mostly ready for commit
to the port, and this too will let you interconnect virtual machines
at high speed without fiddling with bridges, tap or other slow solutions.

The API for applications is unchanged, so you can use the code
in tools/tools/netmap (which i will update soon) on the VALE ports.

This commit also syncs the code with the one in my internal repository,
so you will see some conditional code for other platforms.
The code should run mostly unmodified on stable/9 so people interested
in trying it can just copy sys/dev/netmap/ and sys/net/netmap*.h
from HEAD

VALE is joint work with my colleague Giuseppe Lettieri, and
is partly supported by the EU Projects CHANGE and OPENLAB