Revision 1702305

[Reverted in 1702410]

APR file based streams shall not superficially support read or write
functions when we know that the APR file itself does not.  Instead,
they will now return a "not supported" stream error.

Note that this check is not perfect for arbitrary APR file handles
(may enable more functions than the handle actually supports) but
works correctly for our STD* streams and the files opened through
our svn_io_* API.

* subversion/libsvn_subr/stream.c
  (make_stream_from_apr_file): Set read and write functions conditionally.
  (svn_stream_from_aprfile2): Determine r/w capabilities to the degree
                              we can based on the APR API.
  (svn_stream_for_stdin,
   svn_stream_for_stdout,
   svn_stream_for_stderr): Update constructor callers.