Build kernel module w/ DKMS

[Neil T. Dantam]

To rebuild the kernel module when the user installs a new kernel, the
latest commits to the "kernel" branch at https://github.com/golems/ach
build the module via DKMS (Dynamic Kernel Module Support) and produce
a Debian package to do so as well. I think this is the least painful
way to maintain/install an out-of-tree kernel module.

If DKMS is installed (`sudo apt-get install dkms`), then `./configure
&& make && make install` will build/install the module via DKMS.
Details in INSTALL file. Can also build debian packages from the
source tree with `debuild -us -uc`.

I don't see any compelling advantages to duplicating this work in the
CMake build, so unless anyone prefers otherwise, I will deprecate
CMake as a build option.

I also made some progress on combining common code between user and
kernel space.

Still TODO:
* More testing
* Combine common user/kernel code in ach_put() and ach_get()

I have to context-switch now to setting up the Baxter at Rice, but with
that going will probably do some real-world testing.

Cheers,
-ntd