mod_slotmem_shm: follow up to r1831869 (check persistent files).

Since persistent files are also reused on stop/start, we must ensure that
they match the same descriptor when reused on the next startup, so add it
to integrity metadata.

Also, the descriptor being the first field in the SHM, we don't need to
copy on the stack it in several places, and can handle it as a pointer.



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1831871 13f79535-47bb-0310-9956-ffa450edef68