Give openafs-dbserver its own doc directory

Give openafs-dbserver its own doc directory

* Give openafs-dbserver its own documentation directory rather than
  linking it to openafs-client, allowing relaxation of the versioned
  dependency on openafs-client to Recommends.  (However, openafs-client
  will still have to be installed for the openafs-fileserver init
  script, which uses the bos binary, so will still be pulled in by
  dependencies.)