Enable DKMS support

Enable DKMS support

* Provide an openafs-modules-dkms package that uses DKMS to dynamically
  build kernel modules for the local kernel and document this option in
  README.modules.  This is an alternative to installing
  openafs-modules-source and building kernel modules for each new kernel
  with module-assistant or make-kpkg.  (Closes: #168852, #525131)
  (LP: #52786, #364426, #369875)
* Have openafs-client recommend at least the current source version of
  openafs-modules-source or openafs-modules-dkms instead of any old
  version.  While the API probably hasn't changed, it's best practice to
  keep a relatively close match between the userspace andn the kernel
  module version.  (LP: #369875)