Copy debian_linux python code in here

Copy debian_linux python code in here

Add a copy of the contents of
/usr/share/linux-support-4.15.0-2/lib/python/debian_linux from the
linux-support-4.15.0-2 package.

Also, remove the KERNELVERSION from rules.defs and the code in the
debian_xen that checks for the old location of the code and fixup
cleaning in debian/rules.

Now, we don't have to annoy everyone any more with missing
linux-support-x.y.z packages, which change every few months, in order to
be able to work with this source package.

This is a first step in the process of cleaning up and simplifying this
part of the packaging.