Switch the prefork MPM over to the new pollset interface.

* server/mpm/prefork/prefork.c:
  (listensocks[]): removed. not required.
  (child_main): stop using listensocks[] in favor of an apr_pollset_t. if
    there is just one listener, then always refer to that listener's
    listen_rec for further processing, otherwise poll and pick up the
    listen_rec pointer from the polling structure's client_data. tweaked
    the round-robin for the new pollset API (and documented).
    cleaning: removed 'offset' which is obviated by the use of a pointer
      to a listen_rec. tighten the block-scope of several variables.
      switch to use a 'status' variable name rather than three separately
      named variables. rename n->numdesc to be more obvious.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@101799 13f79535-47bb-0310-9956-ffa450edef68