Add a test for svn_fs_try_process_file_contents.

* subversion/tests/libsvn_fs/fs-test.c
  (process_file_contents_baton_t,
   process_file_contents): New processing callback & baton type.
  (test_zero_copy_processsing): New test.
  (test_funcs): Register the new test.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1636873 13f79535-47bb-0310-9956-ffa450edef68
Docstring fix.  No functional change.

* subversion/include/svn_io.h
  (svn_stream_skip): Add missing "@since" statement.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1636874 13f79535-47bb-0310-9956-ffa450edef68