netlink-socket: Adapt to Windows and MSVC.

Add two functions set_sock_pid_in_kernel and portid_next. This will allow
the channel identification for the kernel extension to send back messages.

Replace send with WriteFile equivalent and ignore nl_sock_drain for the moment
under MSVC.

Replace sendmsg and recvmsg with ReadFile and WriteFile equivalents.

On MSVC put in handle instead of fd(sock->fd becomes sock->handle).

Creation of the netlink socket will be replaced by CreateFile equivalent.

Add MAX_STACK_LENGTH for MSVC.  This will be our maximum size for on-stack
copy buffer.

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