On svn-auth-x509 branch, add svn_utf_string_to_utf8_ex() API.

I'll need this API to convert BMPString and UniversalString types to UTF-8.
They are UCS-2BE and UCS-4BE respectively, which means they may have NUL bytes
and so we need a function to convert a counted length string of a given encoding
to UTF-8, which we don't have right now.

* subversion/include/svn_utf.h
  subversion/libsvn_subr/utf.c:  Add svn_utf_string_to_utf8_ex().


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/svn-auth-x509@1612748 13f79535-47bb-0310-9956-ffa450edef68