Introduce a null-pointer constant that we can use as a termination sentinel
in variable argument lists. This avoids the ugliness of writing
"(char *) NULL" all over the place.

* subversion/include/svn_types.h (SVN_VA_NULL): New symbol.

* subversion/include/svn_dirent_uri.h (svn_dirent_join_many),
  subversion/include/svn_path.h (svn_path_join_many),
  subversion/include/svn_xml.h (svn_xml_make_open_tag):
   Update docstrings, mentioning SVN_VA_NULL as the argument list terminator.

[Lots of implementation files]: Replace typecast-NULL with SVN_VA_NULL in
function calls with variable argument lists.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1536307 13f79535-47bb-0310-9956-ffa450edef68