Add a Windows-specific conversion function from UTF-18 to UTF-8.
Use it in JNIUtil.cpp instead of the APR internal function.

* subversion/libsvn_subr/win32_xlate.h
  (svn_subr__win32_utf8_to_utf16): Declaration moved from here to ...
* subversion/include/private/svn_subr_private.h
  (svn_subr__win32_utf8_to_utf16): ... here.
  (svn_subr__win32_utf16_to_utf8): New prototype.
* subversion/libsvn_subr/win32_xlate.c
  (svn_subr__win32_utf8_to_utf16): Calculate the length of the source
   string only once, and null-terminate it just before returning it.
  (svn_subr__win32_utf16_to_utf8): Implement here.

* subversion/bindings/javahl/native/JNIUtil.cpp: Do not include the private
   APR header arch/win32/apr_arch_utf8.h.
  (JNIUtil::JNIGlobalInit): Use svn_subr__win32_utf16_to_utf8.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1534037 13f79535-47bb-0310-9956-ffa450edef68