Build shadow header files if there is no unified kernel header tree

Build shadow header files if there is no unified kernel header tree

Debian's 2.6.29 packages no longer include symlinks from the
architecture-specific header tree to the common header tree; instead,
they use the kbuild machinery with multiple include paths to overlay
the two header files.  Adjust both the Autoconf probes to always use
kbuild and the script that generates the build tree.  For the latter,
when we don't have a unified header tree, we need to generate headers
in the place where the OpenAFS source expects them that include the
linux/* headers.

Patch from Aaron M. Ucko.