Haiku package: Add a data/empty directory

Can be used when a path to a directory is needed that is certainly
emtpy.
Remove B_COMMON*_DIRECTORY and kCommon*Directory constants

* This does intentionally break source compatibility, so that a review
  of concerned code is forced.
* Binary compatibility should be maintained in most cases. The values
  of the constants for the writable directories are now used for the
  writable system directories. The values for the non-writable
  directories are mapped to "/boot/system/data/empty/...", an empty or
  non-existent directory, so that they will simply be skipped in search
  paths. Only code that explicitly expects to find something in a
  B_COMMON_* directory, will fail.