Add some is-buffered support to lazy-open streams.

* subversion/libsvn_subr/stream.c
  (is_buffered_lazyopen): New.
  (svn_stream_lazyopen_create): Set is-buffered function.

* subversion/include/private/svn_io_private.h
  (svn_stream__is_buffered): Note limitation.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1461770 13f79535-47bb-0310-9956-ffa450edef68