Use /lib/modules/`uname -r`/source as the default kernel source

Use /lib/modules/`uname -r`/source as the default kernel source

* In openafs-modules-source, use /lib/modules/`uname -r`/source as the
  default path to the kernel headers rather than /usr/src/linux.
  Current kernel header packages will create an appropriate symlink
  there.  This default is only used when neither module-assistant nor
  kernel-package are used.