Add DKMS support

[evelikov]

Build tested only

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@lampotakl feel free to give this a try. It works properly on my end.

[evelikov]

Fwiw you do not need to explicitly run "dkms build" and then "dkms install". The latter will do both, as needed.

Practically similar to how the kernel behaves, all (non-buggy) programs using make and many other tools.

[evelikov]

If it were me, I would remove all the module building bits from the top-level Makefile.am since wrapping the kernel build like that usually leads to grief. Just learned about this project, so take this with a truck-full of salt.

[lampotakl]

It seems that it's not enough to just build it like this. The resulting kernel module file has small size as well (13 kB instead of 500kB) and it's not working. I guess that the autoconf/autotools are adding some xtables libraries or something to this module for it to work.
I'm trying to delve into how the original build works for now.

[lampotakl]

On second thought, just running make using this Makefile without DKMS will generate correct kernel module file.
So it's a problem somewhere in DKMS.