debian: Apply Ubuntu patch to add DKMS support.

debian: Apply Ubuntu patch to add DKMS support.

I tested that installing openvswitch-datapath-dkms worked OK on my own
Debian machine.

The bulk of this patch is taken from downstream Ubuntu DKMS support written
by Chuck Short <zulcss@ubuntu.com>, version 1.2.0-1ubuntu1.  I made the
following changes:

  * Update debian/.gitignore.

  * Update debian/automake.mk.

  * Correct description in debian/control (it was a cut-and-paste from
    the openvswitch-datapath-source description without editing).

  * Fix up for --with-l26 to --with-linux and datapath/linux-2.6 to
    datapath/linux transitions.

CC: Chuck Short <zulcss@ubuntu.com>
CC: Dave Walker <DaveWalker@ubuntu.com>
Acked-by: Simon Horman <horms@verge.net.au>