Move 'svn_client_mtcc.h' into the 'private' subdirectory, so we don't make
it public in 1.9.

It's not considered stable enough in its current form. See the email thread
"The new svn_client_mtcc API" started on 2014-07-29 by Julian Foad, archived
at e.g. <http://svn.haxx.se/dev/archive-2014-07/0138.shtml>.

* subversion/include/svn_client_mtcc.h
  Move ...
* subversion/include/private/svn_client_mtcc.h
  ... to here, and rename all symbols: s/svn_client_mtcc/svn_client__mtcc/.

* subversion/libsvn_client/mtcc.c,
  subversion/svnmucc/svnmucc.c,
  subversion/tests/libsvn_client/client-test.c,
  subversion/tests/libsvn_client/mtcc-test.c
    Adjust the '#include', and track the renamed symbols.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1614599 13f79535-47bb-0310-9956-ffa450edef68