Add an optional prefix parameter to the Windows-specific UTF-8 <-> UTF-16
conversion functions.

* subversion/include/private/svn_utf_private.h
  (svn_utf__win32_utf8_to_utf16, svn_utf__win32_utf16_to_utf8):
   Added prefix argument and updated docstrings.
* subversion/libsvn_subr/utf.c
  (svn_utf__win32_utf8_to_utf16, svn_utf__win32_utf16_to_utf8):
   Implemented prefix handling.
* subversion/bindings/javahl/native/JNIUtil.cpp,
  subversion/libsvn_subr/cmdline.c,
  subversion/libsvn_subr/config_win.c,
  subversion/libsvn_subr/nls.c: Updated all callers.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1534953 13f79535-47bb-0310-9956-ffa450edef68