* configure.in: expose DBUS_DATADIR

* configure.in: expose DBUS_DATADIR

* bus/config-parser.c: add the standard_session_servicedirs element
  to the parser
  (bus_config_parser_content): process the standard_session_servicedirs
  element by getting the standard directories from sysdeps and merging
  them into the service directory list
  (test_default_session_servicedirs): make sure we get what we expect

* bus/session.conf.in: replace the servicedir tag with the
  standard_session_servicedirs tag

* dbus/dbus-list.h: remove the typedef of DBusList and place it in
  dbus-sysdeps.h to avoid circular header dependencies

* dbus/dbus-sysdeps.h: add the typedef of DBusList

* dbus/dbus-sysdeps-unix.c (split_paths_and_append): utility function
  which takes a string of directories delimited by colons, parses them
  out, appends a suffix and puts them in a list ignoring empty elements
  (_dbus_get_standard_session_servicedirs): returns the standard
  directories for a session bus to look for service activation files
  on Unix which includes the XDG_DATA_HOME, XDG_DATA_DIRS and
  DBUS_DATADIR directories

* test/data/valid-config-files/many-rules.conf: add the
  standard_session_servicedirs tag to the valid config file tests