Optimize config file parser a bit.

* subversion/libsvn_subr/config_file.c
  (svn_config__parse_file): Use unbufferd I/O since config file parser 
   uses own buffering.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1486897 13f79535-47bb-0310-9956-ffa450edef68