Refactor hash access in svn_config_t code.  Provide a utility function
that minimizes the overhead for case-sensitive access.

* subversion/libsvn_subr/config.c
  (get_hash_value): new utility function
  (find_option,
   svn_config_has_section): use it to simplify code & speed things up

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1538068 13f79535-47bb-0310-9956-ffa450edef68