Simplify win32 stream fix for svnadmin.  [Cleanup of 2189].
Simplify win32 stream fix for svnadmin.  [Cleanup of 2189].

* svnadmin/main.c 
  (create_stdio_stream): simplify by taking a function pointer.  fix comment.
  (main):  pass apr_file_open_std* functions directly to helper.

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion