Refactor svnauth code in preparation for future enhancements.
No functional change.

* subversion/svnauth/svnauth.c
  (list_credentials_baton_t): Rename to ...
  (walk_credentials_baton_t): ... this. Add a boolean 'list' member that
   toggles the listing of credentials.
  (list_credentials): Rename to ..
  (walk_credentials): ... this. Simplify by calling new helper functions.
   Only list credentials of walk_baton->list is TRUE.
  (match_credential, list_credential): New helper functions, factored
   out from walk_credentials().
  (subcommand_list): Set walk_baton->list, and track baton/function renames.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1514322 13f79535-47bb-0310-9956-ffa450edef68