Allow implementations of the svn_config_walk_auth_data callback to
signal to this function that they no longer wish to be called.

* subversion/include/svn_config.h
  (svn_config_auth_walk_func_t): Note that SVN_ERR_CEASE_INVOCATION is
    a magic return value.

* subversion/libsvn_subr/config_auth.c
  (svn_config_walk_auth_data): Accept SVN_ERR_CEASE_INVOCATION from
    the callback as a graceful "stop calling me!" indication.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1466496 13f79535-47bb-0310-9956-ffa450edef68