dpif-linux: Rename dpif-netlink; change to compile with MSVC.

The patch contains the necessary modifications to compile and also to run
under MSVC.

Added the files to the build system and also changed dpif_linux to be under
a more generic name dpif_windows.

Added a TODO under the windows part in case we want to implement another
counterpart for epoll functions.

Signed-off-by: Alin Gabriel Serdean <aserdean@cloudbasesolutions.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>