Following up on r1561688, remove the automatic use of a full read as default
implementation for a possibly partial read. Provide a default full read
implementation for code that only implements partial reads.

* subversion/include/svn_error_codes.h
  (SVN_ERR_STREAM_DATA_AVAILABLE_NOT_SUPPORTED): Rename to...
  (SVN_ERR_STREAM_NOT_SUPPORTED): ... this instead of adding yet another error.

* subversion/include/svn_io.h
  (svn_stream_set_read2,
   svn_stream_set_read): Tweak comments.
  (svn_stream_read2): Document not implemented behavior.
  (svn_stream_data_available): Update error code.

* subversion/libsvn_ra_svn/cyrus_auth.c
  (sasl_read_full_cb): Remove function.
  (svn_ra_svn__enable_sasl_encryption): Pass null instead of stub.

* subversion/libsvn_ra_svn/streams.c
  (sock_read_full_cb): Remove function.
  (svn_ra_svn__stream_from_sock): Pass NULL instead of stub.

* subversion/libsvn_subr/deprecated.c
  (svn_stream_set_read): Update caller with new rules.

* subversion/libsvn_subr/stream.c
  (full_read_fallback): New function.
  (svn_stream_read2): Return documented error instead of assert when not set.
  (svn_stream_read_full): Fall back to multiple calls to svn_stream_read2
    not set.
  (data_available_handler_apr): Update error code.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1562077 13f79535-47bb-0310-9956-ffa450edef68