packaging:  redo kernel pkg deps; sort out python deps & substvars

- 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

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

- 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.

- Migrate from deprecated `python-support` to `dh_python2`
  - Require `python (>= 2.6.6-3~)`, according to docs
  - Ignore 'shebangs' to silence errors; shebangs are correct anyway

- 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

- Remove extraneous `rtai-modules-@KVER@` dep from RTAI modules pkg,
  and `libxenomai1` dep from Xenomai modules pkg