Rework debian/configure

(This commit best read using 'git show -C')

The source configuration script needs to be runnable even in
environments where package build dependencies are absent.

Because people building packages ought to know what they're doing, the
checks for build deps are removed, and all configuration now comes
from the command line.  After all, builds will fail with meaningful
errors if build dependencies aren't met.

An assumption made is that Xenomai (and RTAI, someday) kernel packages
are the Zultron/Dovetail Automata packages.  This is convenient, since
those packages are based on upstream Debian packaging, and the way
those are set up, it's trivial to generate the right header directory
path, Build-Depends:, and Depends:.

A side-effect of this assumption is it's trivial to tell the Universal
Build to build against multiple kernel versions with one LCNC kmodule
package for each.

By default, only POSIX and RT_PREEMPT threads are enabled.  If the
package is built from scratch without first running debian/configure,
debian/configure will automatically run and configure those defaults.

- Simplify debian/configure structure
  - Command-line args correspond directly to per-flavor config functions
  - All configs for a flavor go into one simple function
  - Just two small utilities to edit the debian/rules file
- Enable multiple, per-kernel version module pkgs per kthreads flavor
- Allow POSIX threads to be disabled
- Enable RT_PREEMPT by default
- Rename non-template *.files.in to *.files
- Add usage() function
- Prune .gitignore