openafs_1_8: init at 1.8.0 (#41889)

- Introduce new "server" output holding the server binaries
 - Adapt tsmbac.patch to new build environment
 - Adapt openafs nixos server module accordingly
 - Update upstream CellServDB: 2017-03-14 -> 2018-05-14
 - Introduce package attributes to refer to the openafs packages to use for
   server, programs and kernel module