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
follow upstream Debian package naming.  The way those are set up, it's
reasonably trivial to generate the right header directory path,
Build-Depends:, and Depends:.  The only tricky part is the kernel
'flavour' is included in the package name, which requires some arch
hardcoding hackery to avoid great complication; the assumption here is
that the list of architectures won't change often.

Assuming Debian package naming makes it easy 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.

- 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
- Compress source package .orig tarball with bz2 by default