Be a bit more friendly to our api users by using a common initializer function
for svn_dirent_t (used for ra_stat and ra_get_dir). This initializes all values
to their defined as unavailable value, instead of slightly different values
between the different ra layers depending on availability and applicability.

* subversion/include/svn_types.h
  (svn_dirent_t): Add note.
  (svn_dirent_create): New function. (Beter late than never)

* subversion/libsvn_ra_local/ra_plugin.c
  (svn_ra_local__get_dir): Use svn_dirent_create instead of pcalloc.

* subversion/libsvn_ra_serf/serf.c
  (path_dirent_walker): Use svn_dirent_create instead of pcalloc.

* subversion/libsvn_ra_svn/client.c
  (ra_svn_get_dir,
   ra_svn_stat): Use svn_dirent_create instead of pcalloc.

* subversion/libsvn_repos/repos.c
  (svn_repos_stat): Use svn_dirent_create instead of pcalloc.

* subversion/libsvn_subr/types.c
  (svn_dirent_create): New function.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1434875 13f79535-47bb-0310-9956-ffa450edef68