Add ap_slotmem_attach() to the slotmem_storage_method.

Cut mod_sharemem.c in 2 so that its features could be
used outside httpd.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/httpd-proxy-scoreboard@425734 13f79535-47bb-0310-9956-ffa450edef68