mpm-query hook: distinguish between DECLINED and APR_ENOTIMPL so that

. ap_mpm_query() can return APR_EGENERAL if called too early (for debugging a module)
. some hypothetical module which implements the mpm-query hook can bypass the MPM
  with APR_ENOTIMPL


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@760864 13f79535-47bb-0310-9956-ffa450edef68