Add new svn_config_get_tristate() to read tristate option from config.

* subversion/include/svn_config.h
* subversion/libsvn_subr/config.c
  (svn_config_get_tristate): New.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1436347 13f79535-47bb-0310-9956-ffa450edef68