Add some more constructors, accessors and tests for spillbufs.

* subversion/include/private/svn_subr_private.h
  (svn_spillbuf__get_memory_size, svn_spillbuf__get_filename,
   svn_spillbuf__get_file, svn_spillbuf__reader_create_extended,
   svn_stream__from_spillbuf_extended): New functions.
* subversion/libsvn_subr/spillbuf.c
  (svn_spillbuf__get_memory_size, svn_spillbuf__get_filename,
   svn_spillbuf__reader_create_extended): Implement.
  (stream_from_reader): New private helper function.
  (svn_stream__from_spillbuf, svn_stream__from_spillbuf_extended):
   Implement using stream_from_reader.

* subversion/tests/libsvn_subr/spillbuf-test.c
  (test_spillbuf__reader, test_spillbuf_reader_spill_all,
   test_spillbuf__stream, test_spillbuf_stream_spill_all,
   test_spillbuf__file_attrs, test_spillbuf_file_attrs,
   test_spillbuf_file_attrs_spill_all): New tests and helpers.
  (test_funcs): Add new tests.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1509137 13f79535-47bb-0310-9956-ffa450edef68