Make the svn_client_cat3 API consistent with the rest of the svn_client
space by not admitting to the existence of entry or WC props.

* subversion/include/svn_client.h (svn_client_cat3):
   Update the docstring to not mention entry props at all.
* subversion/libsvn_client/cat.c (svn_client_cat3):
   Filter entry and WC props from the set that came from the
   RA layer, and always set the output parameter for returned
   properties just before streaming the file contents.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1603501 13f79535-47bb-0310-9956-ffa450edef68