Fix issue 4504: writing to read-only auth cache files.

* subversion/libsvn_subr/config_auth.c
  (svn_config_write_auth_data): Use temporary file and rename to handle
   permissions problems, note some questions about existing behaviour.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1596539 13f79535-47bb-0310-9956-ffa450edef68