packaging:  migrate to dh_python2; substvars warnings; python versions

- Migrate from deprecated python-support to dh_python2
  - Require `python (>= 2.6.6-3~)`, according to docs

- Fix substvars `unused/unknown substitution variable` warnings
  - Sort out `shlibs:Depends`, `python:Versions` variables
  - Remove `python-version`; see below
  - Remove `preempt-rt`; see below

- Clean up python deps
  - Remove `python (>= 2.6), python (<< 2.8)` deps
    - These duplicate a subset of the auto-generated `python:Depends`
      variable
  - Remove `${python-version}` from deps
    - These packages exist in Wheezy and later with unversioned names

- Convert `preempt-rt` module package kernel deps from `substvars`
  - Instead use architecture restriction syntax to select the right
    kernel
  - Removal from `debian/substvars` also removes warnings for
    unrelated packages

- Remove `debian/substvars`
  - The two variables it had contained, `preempt-rt` and
    `python-version`, are now removed
  - Remove related code in rules.in
    - `debian/substvars` target, `PYTHON_VERSION` detection, etc.

- Remove extraneous `rtai-modules-@KVER@` dep from RTAI modules pkg
DO NOT MERGE Debian packaging:  Fix kernel package dependencies DO NOT MERGE

*** DO NOT MERGE ***
This patch WILL BREAK with @kinsamanka's RPi kernel packaging and
needs discussion between RT kernel packagers.
*** DO NOT MERGE ***

Re-introduce Debian-style kernel package dependencies with
architecture restrictions in the `machinekit-xenomai` thread packages.

This is intended to re-enable automatic kernel installation with a MK
flavor thread package, with no need for a separate `apt-get`
command to install kernel packages.