Fix a problem whereby multiple MMapFile directives would cause a segfault

Fix a problem whereby multiple MMapFile directives would cause a segfault
on startup.

mod_file_cache keeps a hash table in the cmd->pool and puts an entry in
that hash table for each of its files and mmaps, all of which are opened
into cmd->pool.  But it registered a cleanup on cmd->pool that would walk
the hash table and close each file and delete each mmap, even though by
the time that happened those things would have been done already anyway
by the files' and mmaps' own cleanups on cmd->pool.  So it was deleting
mmaps that were already cleaned up and closing files that were already
cleaned up in all cases.  This has never been valid... amazed it ever
worked.  But apparently the true bogosity wasn't revealed until the new
mmap cleanup code went into APR.

PR: 16313

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@98463 13f79535-47bb-0310-9956-ffa450edef68