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 easy 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 otherwise avoid great complication; the hope
here is that the list of architectures won't change often.

The result is the Universal Build builds against possibly multiple
kernel versions, and the Debian packaging builds one kmodule flaver
package for each.

By default, all threads are disabled; package builders MUST enable
thread package builds by adding flags to the debin/configure command
line.  See `debian/configure -h` for details.

Summary of changes:
- 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 a few small helper utilities
- Enable multiple, per-kernel version module pkgs per kthreads flavor
- All/any flavors may be enabled or disabled
- Rename non-template *.files.in to *.files
- Add usage() function
- Update .gitignore
- Bonus:  compress source package .orig tarball with bz2 by default